Ahhhh ok it makes sense now hehe sorry!
Ok this is a difficult one to give advice because everyone is so individual, what may work for one may be a huge turn off for another. (I am thinking, for example, that some people just love to hear new ideas, the dirtier the better, the more discriptive the better. While others get embarrassed or turned off if approached in certain ways)
So I guess in this situation you know better than any of us about the best way to approach this situation with her. However I guess we can give you ideas or different approaches and you can work out which would work best.
If this was me, I would probably start by slowly building up some compliments, romance, special treats and building up her confidence a little sexually. I am not saying she isnt confident, or that you dont make the effort. But you know, when you are with someone for a long time, we begin to stop making that extra special effort to make our lover feel REALLY sexy about themselves. As women, when we feel really sexy and confident and appreciated, we are much more likely to want to give back. The whole "give and take" thing. Almost a bit like reverse psychology here. You want something special from her, make the effort for her first and her mindset will be much more open to try new things. That would be my first tip.
Second tip I would give is to make sure, when you bring this up with her, to talk about this like it is HER you are going crazy for, and the fantasy is just an "extra" This is probably true anyway of course, but women are sensitive and if you word it in such a way that you make it personal to her, you know like:
"I was thinking the other day about that time YOU were wearing my shirt, mmm YOU looked so amazing,
I loved the way YOUR......
I would love to see YOUR sexy body in....."
You get the idea? Rather than "remember when you USED to do...." (which can sound like an accusation, like, why are you not doing that now, you make no effort anymore, i dont fancy you as much now because of this. (Yes women read too much into things lol)
Or saying "I really have this fantasy about a woman dressing up as...." (again this approach had the tendancy, with an over thinking or insecure or unsure woman (unsure to try new things I mean) to maybe make her feel that it isnt her you are fantasising about, rather, just using her as your doll for your particular fantasy. This might put her off. (Ahhh the male-female communication differences abound :D)
So this would be my advise. if you are sure that bringing it up right now would turn her off. if you have already brought it up and it has negative results then this is it: Do some groundwork first. Make an effort to get into her mind, maybe find out if there is anything you can do to please her more or to play some games she might be interested in. Try being a bit more "like you used to be" at the beginning, and make that extra special effort for her. (Hopefully this should, in return, make her want to do more for you) Of course dont go too overboard with it and then come out with your fantasy or she may just suspect you only made the effort to get what you want. It has to be genuine ofc.
Compliment her skills/techniques, the way she turns you on (for example a text or email or if brave enough just say "wow what you did last night was amazing" etc then gradually start hinting, in the way I mentioned above, and in a light hearted, jokey, no pressure kind of way (example: "oooh last night when you did XYZ you had me imagining you as my naughty secretary wink wink big grin") You may find that the ego boost you are giving her this way will inspire her to go out and get the outfit herself! (She will hopefully be thinking "wow, I really turned him on! and he was imagining that, well I am gunna show HIM how naughty i can really be")
To sum up my best advice: If you want to receive: give.....give give give! :D
Of course all the above is just my opinion. I am sure that some women would say that all of the above is rubbish and wouldnt work for them (which is what I mean by you knowing her best, and us all being different) Maybe she actually really really detests the idea of doing XYZ and if thats the case you will only push her away further by pressuring her on the subject. I am sure you can appreciate that if the roles were reversed. In that case I would advise leaving it alone and just keeping it as a fantasy.
Good luck! Hope you get your sexy secretary fantasy x