Main artery (5)
'main artery' is the definition.
(largest artery)
This is all the clue.
(Other definitions for aorta that I've seen before include "This carries blood from the heart" , "Bloody channel" , "Main artery of body" , "A rota (anag)" , "Great artery in body" .)
(Other definitions for trunk road that I've seen before include "Arterial thoroughfare" , "major way" .)