Stimulating hormone (9)
I believe the answer is:
adrenalin
'stimulating hormone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for adrenalin that I've seen before include "Hormone affecting metabolism" , "Booster" , "that pumps you up" , "stimulant" , "Hormone that stimulates the nervous system" .)