Groups are playing with singular resentment (4,6)
I believe the answer is:
sour grapes
'resentment' is the definition.
(I have seen 'Jealous resentment' mean 'sour grapes' so perhaps 'resentment' could also mean 'sour grapes')
'groups are playing with singular' is the wordplay.
'playing with' indicates an anagram.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'groups'+'are'='groupsare'
'groupsare' with its letters mixed up with 's' can make 'SOUR GRAPES'.
(Other definitions for sour grapes that I've seen before include "Affected disdain" , "Disparagement of something one wants but cannot attain" , "Feigned dislike" , "Derision of the unattainable" , "Disparagement of something one can't have oneself" .)