Supporter runs away from mass meeting (4)
I believe the answer is:
ally
'supporter' is the definition.
(I've seen this before)
'runs away from mass meeting' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'away from' indicates named letters should be taken away.
'mass meeting' becomes 'rally' (synonyms).
'rally' with 'r' taken out is 'ALLY'.
(Other definitions for ally that I've seen before include "Fighting partner" , "Cooperating country" , "One in cooperation with another" , "Join forces with" , "Someone with whom one is in league" .)