A girl is wrong (5)
I believe the answer is:
amiss
'is wrong' is the definition.
('amiss' can be a synonym of 'wrong')
'a girl' is the wordplay.
'girl' becomes 'miss' (I've seen this before**).
'a'+'miss'='AMISS'
(Other definitions for amiss that I've seen before include "Faulty" , "Mistaken" , "Wrong - awry" , "inappropriately" , "Taking wrong turn" .)