Bodily chemical involved in mood regulation (9)
I believe the answer is:
serotonin
(Other definitions for serotonin that I've seen before include "Chemical compound in body" , "Compound acting as a neurotransmitter" , "Neurotransmitter compound; soon inert (anag.)" , "Compound present in serum - ironstone (anag)" , "Neurotransmitter hormone, involved in depression say" .)