Bolsheviks (4)
I believe the answer is:
reds
'bolsheviks' is the definition.
('red' can be a synonym of 'bolshevik')
This is all the clue.
(Other definitions for reds that I've seen before include "Scarlet and crimson, say" , "Scarlet communists" , "These are not right" , "First snooker balls to be potted" , "Colourful communists" .)