Ready to take small groups (4)
I believe the answer is:
sets
'groups' is the definition.
(I've seen this in other clues)
'ready to take small' is the wordplay.
'ready to' becomes 'set' (synonyms).
'take' 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 "Hardens" , "Plants" , "Classes" , "Groups of things of the same kind" , "Major divisions of tennis match" .)