A colourful leftist (3)

I believe the answer is:

red

I'm a little stuck... Click here to teach me more about this clue!

'leftist' is the definition.
(red can informally mean communist)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for red that I've seen before include "Very angry, seeing this" , "lefty" , "Bordeaux claret, for instance" , "Colour for a blusher" , "Primary colour" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app