Colleague right to leave mass meeting (4)
I believe the answer is:
ally
'colleague' is the definition.
(ally can mean a colleague or associate)
'right to leave mass meeting' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to leave' 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 "Confederate; supporter" , "State formally cooperating with another" , "Friend and colleague" , "Friendly nation" , "One on one's side" .)