It can react jerkily (4)

I believe the answer is:

knee

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'it can react jerkily' 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 knee that I've seen before include "Technical name for this joint is genu" , "Hinge joint" , "Thigh/shin connector" , "Capped joint?" , "Joint protected by the patella" .)

I've seen this clue in the Evening Standard.

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