Most of tax is on a gold vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(blood vessel)
'most of tax is on a gold' is the wordplay.
'most of' means to remove the last letter.
'is on' is a charade indicator (letters next to each other).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'tax' with its last letter taken off is 'ta'.
'a'+'or'='aor'
'ta' after 'aor' is 'AORTA'.
(Other definitions for aorta that I've seen before include "Main human artery" , "Large artery of the heart" , "Major artery of the body" , "Blood-carrier from a heart" , "Heart's great artery" .)