Tip over mine (3)
I believe the answer is:
'mine' is the definition.
(down the pit)
'tip over' is the wordplay.
'over' says the letters should be written backwards.
'tip' written backwards gives 'PIT'.
(Other definitions for pit that I've seen before include "Mine; fruit-stone" , "where orchestra plays" , "Cherry stone" , "Coal mine" , "Coalmine" .)