A girl? Wrong (5)
I believe the answer is:
amiss
'wrong' is the definition.
(I know that wrong can be written as amiss)
'a girl?' is the wordplay.
'girl?' becomes 'miss' (term of address for an unmarried woman).
'a'+'miss'='AMISS'
(Other definitions for amiss that I've seen before include "Not quite right" , "Fallacious" , "error" , "astray" , "(Gone) wrong" .)