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 see 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 found in the answer.
The remaining letters 'mr' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
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" .)