A mine with seats in? (3)

I believe the answer is:

pit

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'a mine' is the definition.
(down the pit)

I can't explain the rest of the clue.

(Other definitions for pit that I've seen before include "Hollow" , "Coal mine" , "Stop to refuel" , "Mine; fruit-stone" , "Sizeable hole in the ground" .)

I've seen this clue in the Evening Standard.

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