In restaurant's fine-ground stimulant (8)
I believe the answer is:
caffeine
'stimulant' is the definition.
(I've seen this before)
'in restaurant's fine-ground' is the wordplay.
'in' indicates putting letters inside.
'restaurant' becomes 'cafe' (cafe is a kind of restaurant).
'ground' indicates an anagram ('grind' the letters into a new form).
'fine' with letters rearranged gives 'fein'.
'cafe' enclosing 'fein' is 'CAFFEINE'.
(Other definitions for caffeine that I've seen before include "it may produce a buzz" , "Stimulant found in tea and coffee" , "Stimulant in tea, etc" , "Stimulating substance found in fine cafe, oddly" , "Stimulant in, eg, cappuccino" .)