Group of left-wingers getting diarrhoea (3,5)
I believe the answer is:
the trots
I believe this clue is a double definition.
'group of left-wingers' is the first definition.
(trot is short for Trotskyite)
'diarrhoea' is the second definition.
(the trots can informally mean diarrhoea)
'getting' is the link.
(Other definitions for the trots that I've seen before include "Those on the Left" , "many movements in a short time?" .)