Nine roots developed into neurotransmitter (9)
I believe the answer is:
serotonin
'neurotransmitter' is the definition.
(serotonin is a kind of neurotransmitter)
'nine roots developed' is the wordplay.
'developed' indicates an anagram.
'nine'+'roots'='nineroots'
'nineroots' is an anagram of 'SEROTONIN'.
'into' is the link.
(Other definitions for serotonin that I've seen before include "Compound in brain" , "Neurotransmitter compound; soon inert (anag.)" , "Neurotransmitter hormone, involved in depression say" , "Compound acting as a neurotransmitter" , "Compound present in serum - ironstone (anag)" .)