Gratuity? Restaurant’s ending with one penny! (3)
I believe the answer is:
tip
'gratuity?' is the definition.
(as in a gratuity to a waiter)
'restaurant's ending with one penny' is the wordplay.
'ending' says to take the final letters (the letter at the end of the word).
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'penny' becomes 'p' (short for pence).
The last letter of 'restaurant' is 't'.
't'+'i'+'p'='TIP'
(Other definitions for tip that I've seen before include "Helpful suggestion" , "Bonus" , "Hint about a horse" , "Point; piece of advice" , "Helpful information; end" .)