Republicans: 'Commies!' (4)
I believe the answer is:
reds
This clue is a double definition.
'republicans' is the first definition.
(traditional colour for US republican party)
'commies' is the second definition.
(red is a kind of commie)
(Other definitions for reds that I've seen before include "These are not right" , "Colours for communists" , "Scarlet and crimson, say" , "Colourful communists" , "Tidies up" .)