Rock up half-heartedly for a joint (5)

I believe the answer is:

elbow

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'for a joint' is the definition.
(joint of the body)

I don't understand how the remainder of the clue works.

(Other definitions for elbow that I've seen before include "sharp turn" , "Sort of grease" , "given to one about to leave" , "push" , "Major arm joint" .)

I've seen this clue in The Telegraph.

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