Key artery (5)
I believe the answer is:
aorta
'key artery' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aorta that I've seen before include "Body's main artery" , "Main arterial vessel, carrying blood to the heart" , "Heart's great artery" , "A rota (anag)" , "Main route from the heart" .)