Pit, colliery (4)
I believe the answer is:
This clue is a double definition.
'pit' is the first definition.
'colliery' is the second definition.
(a colliery is a coal mine)
(Other definitions for mine that I've seen before include "Pit; explosive device" , "Bomb - colliery" , "Donnybrook" , "Dig for minerals like coal" , "I made?" .)