Neurotransmitter on stonier ground (9)
I believe the answer is:
serotonin
'neurotransmitter' is the definition.
(serotonin is a kind of neurotransmitter)
'on stonier ground' is the wordplay.
'ground' is an anagram indicator (grind the letters ).
'on'+'stonier'='onstonier'
'onstonier' is an anagram of 'SEROTONIN'.
(Other definitions for serotonin that I've seen before include "Compound present in serum - ironstone (anag)" , "Chemical compound in body" , "Neurotransmitter hormone, involved in depression say" , "Neurotransmitter compound; soon inert (anag.)" , "Compound in brain" .)