Crossword Genius

Joint that needs grease added; it helps with hard graft (5)

Ross

I believe the answer is:

elbow

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

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

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(Other definitions for elbow that I've seen before include "sack?" , "Nudge" , "Reject" , "Barge" , "part of the body" .)

I've seen this clue in The Irish Times.
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