He's only a quarter of an hour at the pit (5)
I believe the answer is:
miner
'he's' is the definition.
'only a quarter of an hour at the pit' is the wordplay.
I cannot really understand how this works, but
'quarter' could be 'ne' and 'ne' is present in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is located in the answer.
The remaining letters 'mr' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for miner that I've seen before include "Digger for gold or coal, say" , "Collier" , "One works underground" , "Excavator" , "it's the pits, for me" .)