Someone who cross-dresses, i.e dressing outside of gender. Sometimes done to look like a convincing member of the opposite sex ("passing"), sometimes not. Sometimes a secret, sometimes not. Nothing to do with sexuality. Not necessarily to do with desiring a sex change either.
"Don't get strung out by the way I look/Don't judge a book by its cover/I'm not much of a man by the light of day/But by night I'm one hell of a lover."