Small supermarket bag (5)
I believe the answer is:
scoop
'bag' is the definition.
(thesaurus)
'small supermarket' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'supermarket' becomes 'Co-op' (The Co-operative).
's'+'coop'='SCOOP'
(Other definitions for scoop that I've seen before include "Waugh novel; some ice-cream" , "Exclusive news story" , "Exclusive report" , "Dredge up" , "war novel" .)