Main artery of the body (5)
I believe the answer is:
aorta
'main artery of the body' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aorta that I've seen before include "Main artery from heart" , "Tube" , "Main arterial vessel, carrying blood to the heart" , "Great artery in body" , "Main artery of body" .)