I think the boss/secretary trope is just a version of a power dynamic, and in fact that is really the basis of the issue of #metoo. Think about the dynamic between your characters in terms of what power the boss holds over the secretary (she needs the job and he could fire her, he determines her work hours, he determines what her responsibilities are - which could be anything from something demeaning like getting him coffee to something that shows that he respects her abilities like making her responsible for leading some important project). Sex is just another form of power, or it can be. Look for some power differential that inspires you. A very common one is he is the boss but OH MY GOD HE WANTS HER SO BAD HE WILL DO ANYTHING FOR HER but he still acts like an Alpha-hole. Common, but popular!