Silly of son to leave crackers (5)
I believe the answer is:
inane
'silly' is the definition.
(thesaurus)
'son to leave crackers' is the wordplay.
'son' becomes 's'.
'to leave' is a deletion indicator.
'crackers' becomes 'insane' ('insane' can be similar in meaning to 'crackers').
'insane' with 's' removed is 'INANE'.
'of' is the link.
(Other definitions for inane that I've seen before include "Empty" , "Absurd" , "Vapid, fatuous" , "Thick" , "Mindlessly silly" .)