Stimulating substance in coffee (8)
I believe the answer is:
caffeine
'stimulating substance in coffee' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for caffeine that I've seen before include "coffee ingredient" , "Stimulant in, eg, cappuccino" , "Stimulant found in mug or cup?" , "it may produce a buzz" , "Stimulant in drinks" .)