Mine closure when mechanics have to work fast (3,4)
I believe the answer is:
pit stop
'mechanics have to work fast' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'mine closure' is the wordplay.
'mine' becomes 'pit' (down the pit).
'closure' becomes 'stop' (synonyms).
'pit'+'stop'='PIT STOP'
'when' is the link.
Can you help me to learn more?
(Other definitions for pit stop that I've seen before include "for rest of travellers" , "part of the race" , "Break for refuelling" , "place where car can be serviced - with 31a" , "brief service" .)