Hormone secreted in response to stress (9)
I believe the answer is:
adrenalin
'hormone secreted in response to stress' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for adrenalin that I've seen before include "Hormone secreted in response to fear" , "stimulant" , "Booster" , "Metabolism-affecting hormone" , "A secretion" .)