Principal volunteers with popular support (8)
I believe the answer is:
maintain
'support' is the definition.
(thesaurus)
'principal volunteers with popular' is the wordplay.
'principal' becomes 'main' (I've seen this before).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'with' is a charade indicator (letters next to each other).
'popular' becomes 'in'.
'main'+'ta'+'in'='MAINTAIN'
(Other definitions for maintain that I've seen before include "Preserve or insist" , "Claim" , "Uphold, preserve" , "Keep in existence or in repair" , "Look after the upkeep" .)