Dominant volunteers in support (8)
I believe the answer is:
maintain
'support' is the definition.
(thesaurus)
'dominant volunteers in' is the wordplay.
'dominant' becomes 'main' (I've seen this before).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'main'+'ta'+'in'='MAINTAIN'
(Other definitions for maintain that I've seen before include "Keep up - carry on" , "finance" , "Keep - support" , "Keep in a certain state" , "Uphold, preserve" .)