Ruin a jam after smashing a pot (9)
I believe the answer is:
marijuana
'pot' is the definition.
(pot can informally mean marijuana)
'ruin a jam after smashing a' is the wordplay.
'after' is a charade indicator (letters next to each other).
'smashing' indicates an anagram.
'ruin'+'a'+'jam'='ruinajam'
'ruinajam' is an anagram of 'marijuan'.
'marijuan'+'a'='MARIJUANA'
(Other definitions for marijuana that I've seen before include "3 Down" , "Pot" , "Cannabis" , "grass" , "Wacky baccy" .)