A mine with seats in? (3)WASACOLUMNBREAK (3)

I believe the answer is:

pit

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

I cannot understand how the rest of the clue works.

(Other definitions for pit that I've seen before include "Tip into hole" , "Tip into miners' shaft" , "See 9" , "Coal mine" , "Mine; fruit stone" .)

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