Republican leaving mass meetings to supporters (6)
I believe the answer is:
allies
'supporters' is the definition.
(I've seen this before)
'republican leaving mass meetings' is the wordplay.
'republican' becomes 'r' (abbreviation).
'leaving' suggests deleting specific letters.
'mass meetings' becomes 'rallies' ('rally' can be a synonym of 'mass meeting').
'rallies' with 'r' taken out is 'ALLIES'.
'to' acts as a link.
(Other definitions for allies that I've seen before include "They join together as in war" , "Colleagues" , "They are banded together for mutual support" , "They help each other, confederates" , "People or nations joined against an enemy" .)