Ready with small groups (4)
I believe the answer is:
sets
'groups' is the definition.
(a set is a group of things)
'ready with small' is the wordplay.
'ready' becomes 'set' (set can mean ready or prepared).
'with' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'set'+'s'='SETS'
(Other definitions for sets that I've seen before include "Scenery etc. of 11 across productions" , "Hardens" , "Classes" , "Places" , "Scenery etc. of dramatic productions" .)