Brain neurotransmitter (9)
I believe the answer is:
serotonin
(Other definitions for serotonin that I've seen before include "Compound in brain" , "Neurotransmitter hormone, involved in depression say" , "Compound in the blood affecting sleep and depression - soon inert (anag)" , "Compound present in serum - ironstone (anag)" , "Chemical compound in body" .)