An entrance I'd turn in at (4)
I believe the answer is:
adit
'an entrance' is the definition.
(I've seen this before)
'i'd turn in at' is the wordplay.
'turn' indicates an anagram.
'in' means one lot of letters goes inside another.
'id' with letters rearranged gives 'di'.
'di' inserted into 'at' is 'ADIT'.
(Other definitions for adit that I've seen before include "Taking passage" , "Pit access" , "Passageway into a mine" , "Mine access passage" , "Horiz. mine entrance" .)