I'd turn in at the entrance (4)
I believe the answer is:
adit
'the entrance' is the definition.
(I've seen this before)
'i'd turn in at' is the wordplay.
'turn' indicates an anagram.
'in' is an insertion indicator.
'id' with letters rearranged gives 'di'.
'di' going inside 'at' is 'ADIT'.
(Other definitions for adit that I've seen before include "Passageway into a mine" , "Mine access passage" , "Mine opening" , "Means of access to mine" , "Horiz. mine entrance" .)