Centre of the arterial system (5)
I believe the answer is:
heart
'centre' is the definition.
(synonyms)
'of the arterial system' is the wordplay.
'of' indicates the answer is hidden within the clue.
'HEART' is hidden within 'the arterial system'.
(Other definitions for heart that I've seen before include "Decorated (cake)" , "Red card" , "Muscular organ - compassion" , "Cardiac organ" , "Not a club" .)