Forgetful about a girl (6)
I believe the answer is:
remiss
'forgetful' is the definition.
(I've seen this in another clue)
'about a girl' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a girl' becomes 'miss' (term of address for an unmarried woman).
're'+'miss'='REMISS'
(Other definitions for remiss that I've seen before include "Careless concerning a girl?" , "Lacking attention to duty" , "Derelict in duty" , "delinquent" , "Neglectful" .)