A girl or man disguised (5)
I believe the answer is:
norma
'a girl' is the definition.
'or man disguised' is the wordplay.
'disguised' is an anagram indicator.
'or'+'man'='orman'
'orman' anagrammed gives 'NORMA'.
(Other definitions for norma that I've seen before include "Opera - ex-PM's wife" , "The 'Rule' constellation" , "Shearer or Major" , "Bellini's heroine" , "Drawing instrument" .)