Communists (4)
I believe the answer is:
reds
'communists' is the definition.
(red is a kind of communist)
This is all the clue.
(Other definitions for reds that I've seen before include "First snooker balls to be potted" , "Scarlet communists" , "Colourful communists" , "Colours for communists" , "Left-wingers" .)