Caffeine, say — help keeping all OK, if stimulated (8)
I believe the answer is:
alkaloid
'caffeine say' is the definition.
(caffeine is a kind of alkaloid)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for alkaloid that I've seen before include "Plant compound having physiological effect on humans" , "Organic compound with toxic or medicinal property" , "Organic compound having physiological action" , "Any of a group of nitrogenous basic compounds found in plants" .)