... what's wrong with one girl... (5)
I believe the answer is:
amiss
'what's wrong' is the definition.
(synonyms)
'with one girl' is the wordplay.
'with' says to put letters next to each other.
'one' becomes 'a' (a thing is one thing).
'girl' becomes 'miss' (term of address for an unmarried woman).
'a' next to 'miss' is 'AMISS'.
(Other definitions for amiss that I've seen before include "astray" , "Wrong; inaccurate" , "Wrorg" , "Haywire" , "Fallacious" .)