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