Robin's left wing. Bert's a revolutionary (9)
I believe the answer is:
redbreast
'robin's' is the definition.
(synonyms)
'left wing bert's a revolutionary' is the wordplay.
'left wing' becomes 'red' (red can informally mean communist).
'revolutionary' is an anagram indicator.
'berts'+'a'='bertsa'
'bertsa' is an anagram of 'breast'.
'red'+'breast'='REDBREAST'
(Other definitions for redbreast that I've seen before include "Bird" , "One tweets" , "Flapper" , "Type of robin" .)