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