Gold volunteers located under a vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(aorta is a kind of vessel)
'gold volunteers located under a' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'located under' says to put letters next to each other.
'or'+'ta'='orta'
'orta' after 'a' is 'AORTA'.
(Other definitions for aorta that I've seen before include "Trunk route" , "Large artery of the heart" , "Major artery at left of heart" , "it's drawing blood!" , "Main human artery" .)