About 500 are leftist (3)
I believe the answer is:
red
'are leftist' is the definition.
(red can informally mean communist)
'about 500' is the wordplay.
'about' becomes 're' (regarding).
'500' becomes 'D' (Roman numeral for 500).
're'+'d'='RED'
(Other definitions for red that I've seen before include "could be a Marxist" , "Very angry, seeing this" , "See it and become angry" , "Bordeaux claret, for instance" , "'If you see it, perhaps you're angry (3)'" .)