Gratuity — mine sent over (3)
I believe the answer is:
tip
'gratuity' is the definition.
(as in a gratuity to a waiter)
'mine sent over' is the wordplay.
'mine' becomes 'pit' (down the pit).
'sent over' says the letters should be written backwards.
'pit' written backwards gives 'TIP'.
(Other definitions for tip that I've seen before include "End; piece of advice" , "Gratuity; rubbish-dump" , "Dump; extremity" , "single piece of asparagus, perhaps" , "Pointed extremity" .)