Large blood vessels (6)
I believe the answer is:
aortas
'large blood vessels' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aortas that I've seen before include "vessels arrive" , "Large trunk arteries" , "They carry blood from hearts" , "So a rat gets into great arteries" , "Major arteries at left or hearts" .)