Main arteries (6)
I believe the answer is:
aortas
'main arteries' 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 "Large trunk arteries" , "vessels arrive" , "So a rat gets into great arteries" , "Major arteries at left or hearts" , "They carry blood from hearts" .)