Crossword Genius

Free policeman, powerless in pit (8)

Ross

I believe the answer is:

coalmine

I'm a little stuck... Click here to teach me more about this clue!

'pit' is the definition.

'free policeman powerless in' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for coalmine that I've seen is " Colliery".)

I've seen this clue in The Telegraph.
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