Red in revolutionary wrongdoing (4)
I believe the answer is:
trot
'red' is the definition.
(Trotskyite, type of socialist or 'red')
'revolutionary wrongdoing' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'wrongdoing' becomes 'tort' (tort is a kind of wrongdoing).
'tort' backwards is 'TROT'.
'in' is the link.
(Other definitions for trot that I've seen before include "Fairly brisk pace" , "Move briskly" , "Go at steady pace" , "Old communist" , "Fairly fast pace of a horse" .)