Republican supporter in mass meeting (5)
I believe the answer is:
rally
'mass meeting' is the definition.
('rally' can be a synonym of 'mass meeting')
'republican supporter' is the wordplay.
'republican' becomes 'r' (abbreviation).
'supporter' becomes 'ally' (I've seen this before).
'r'+'ally'='RALLY'
'in' is the link.
(Other definitions for rally that I've seen before include "Motor sport event" , "Recover; mass meeting" , "Get-together" , "Recover health" , "Get better; car meet" .)