American gold at capsized vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(blood vessel)
'american gold at capsized' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'capsized' says the letters should be written backwards (in a down clue, the word turns upside down).
'at' back-to-front is 'ta'.
'a'+'or'+'ta'='AORTA'
(Other definitions for aorta that I've seen before include "Major artery of the body" , "Trunk route" , "claret supplier" , "Large trunk artery" , "it's drawing blood!" .)