An organ in the arterial system (3)
I believe the answer is:
ear
'an organ' is the definition.
(ear is a kind of organ)
'in the arterial system' is the wordplay.
'in' says the answer is hidden in the clue.
'EAR' is hidden within 'the arterial system'.
(Other definitions for ear that I've seen before include "Bit of corn" , "Seed-bearing part of corn" , "A corny bit" , "organ with drum" , "Speculator ignoring British" .)