Part of the blood circulation system (4)
I believe the answer is:
vein
'part of the blood circulation system' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for vein that I've seen before include "Speaker's useless" , "Leaf rib; ore layer in rock" , "Vessel for claret" , "Deposit of mineral" , "Distinctive quality" .)