Quick -- little son needs diaper! (6)
I believe the answer is:
snappy
'quick' is the definition.
(thesaurus)
'little son needs diaper' is the wordplay.
'little son' becomes 's' (abbreviation for son).
'needs' means one lot of letters go next to another.
'diaper' becomes 'nappy' ('nappy' can be a synonym of 'diaper').
's'+'nappy'='SNAPPY'
(Other definitions for snappy that I've seen before include "Short-tempered" , "Ratty" , "Irritable, curt; stylish" , "Informally brisk" , "Irritable - quick - fashionable" .)