Collier's entrance (4)
I believe the answer is:
adit
'collier's entrance' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for adit that I've seen before include "Pit access" , "Mine access passage" , "Da, it is a mine entrance" , "Horizontal mine passage" , "Means of access to mine" .)