It's useful in the theatre (in the pit?) (4)

I believe the answer is:

prop

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'pit?' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for prop that I've seen before include "Support - item on stage" , "one playing Rugby" , "A rigid support - 37 Across is one" , "needed at certain stages" , "Back up" .)

I've seen this clue in the Evening Standard.

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