Hormone in the brain with an analgesic effect (9)
I believe the answer is:
endorphin
'hormone in the brain with an analgesic effect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for endorphin that I've seen before include "Opiate-like substance produced in the brain" , "Hormone in the brain with an analgesic effect" , "painkiller" , "Neurochemical with analgesic properties - dinner hop (anag)" .)