Drunk son, crazy about the girl (7)
I believe the answer is:
smashed
'drunk' is the definition.
(I've seen this in another clue)
'son crazy about the girl' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'crazy' becomes 'mad' (synonyms).
'about' is an insertion indicator.
'the girl' becomes 'she' (I've seen this in another clue).
'mad' placed around 'she' is 'mashed'.
's'+'mashed'='SMASHED'
(Other definitions for smashed that I've seen before include "completely sozzled" , "Broke; drunk (colloq.)" , "Affected by drug" , "Broken in pieces" , "Broke into sharp pieces" .)