Supporter Republican dismissed from mass meeting (4)
I believe the answer is:
ally
'supporter' is the definition.
(I've seen this before)
'republican dismissed from mass meeting' is the wordplay.
'republican' becomes 'r' (abbreviation).
'dismissed from' suggests deleting specific letters.
'mass meeting' becomes 'rally' ('rally' can be a synonym of 'mass meeting').
'rally' with 'r' taken out is 'ALLY'.
(Other definitions for ally that I've seen before include "Fighting partner" , "Join by marriage, treaty etc" , "One on one's side" , "Accomplice" , "One co-operating with another" .)