Chief with volunteers in support (8)
I believe the answer is:
maintain
'support' is the definition.
(thesaurus)
'chief with volunteers in' is the wordplay.
'chief' becomes 'main' (I've seen this before**).
'with' says to put letters next to each other.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'main'+'ta'+'in'='MAINTAIN'
(Other definitions for maintain that I've seen before include "Claim" , "Conserve" , "Keep going" , "Continue with an opinion" , "Keep up - carry on" .)