A time to seal off copper mine entrance (4)
I believe the answer is:
adit
'mine entrance' is the definition.
(I've seen this before)
'a time to seal off copper' is the wordplay.
'time' becomes 't' (abbreviation).
'to seal off' is an insertion indicator.
'copper' becomes 'di' (detective inspector, type of copper).
'a'+'t'='at'
'at' placed around 'di' is 'ADIT'.
(Other definitions for adit that I've seen before include "Mine opening" , "Taking passage" , "Mine entrance passage" , "Passageway into a mine" , "Pit access" .)