Rejected gratuity is mine (3)
I believe the answer is:
pit
'is mine' is the definition.
(down the pit)
'rejected gratuity' is the wordplay.
'rejected' says the letters should be written backwards.
'gratuity' becomes 'tip' ('tip' can be a synonym of 'gratuity').
'tip' back-to-front is 'PIT'.
(Other definitions for pit that I've seen before include "Big deep hole in the ground" , "Stop to refuel" , "Cherry-stone" , "Mine-shaft" , "Place for the accompanying orchestra" .)