Free crack in plastic I'd seen (9)
I believe the answer is:
disengage
'free' is the definition.
(I know that free can be written as disengage)
'crack in plastic i'd seen' is the wordplay.
'crack' becomes 'gag' (both can mean a joke).
'in' is an insertion indicator.
'plastic' indicates anagramming the letters (plastic can mean easily moulded).
'id'+'seen'='idseen'
'idseen' is an anagram of 'disene'.
'gag' placed within 'disene' is 'DISENGAGE'.
(Other definitions for disengage that I've seen before include "Withdraw" , "Separate, release, detach" , "Remove (troops) from a conflict" , "free" .)