Woman, or man in disguise (5)
I believe the answer is:
norma
'woman' is the definition.
'or man in disguise' is the wordplay.
'in disguise' is an anagram indicator.
'or'+'man'='orman'
'orman' with letters rearranged gives 'NORMA'.
(Other definitions for norma that I've seen before include "Marilyn Monroe's real first name" , "Girl's name - may be Roman" , "Roman (anag) -- opera" , "Marilyn was properly one" , "Manor (anag.)" .)