It fits in jointly (5)

I believe the answer is:

tenon

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'it fits in jointly' is the definition.
The definition suggests a singular noun which matches the answer.

This is all the clue.

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(Other definitions for tenon that I've seen before include "Saw; joint" , "Piece of wood that fits into a mortise" , "Wooden projection shaped to fit in a mortise joint" , "Mortise counterpart" , "End of a piece of wood shaped to form a mortise joint" .)

I've seen this clue in the Evening Standard.

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