Left-winger to run (4)
I believe the answer is:
trot
I believe this clue is a double definition.
'left-winger' is the first definition.
(short for Trotskyite)
'run' is the second definition.
(trotting is a kind of running)
'to' acts as a link.
(Other definitions for trot that I've seen before include "moderate movement" , "A slowish run, perhaps by horse" , "Run at moderate pace" , "Fairly fast pace of a horse" , "Go at steady pace" .)