Crossword Genius

Joint wretched, get to work (7,4)

Ross

I believe the answer is:

knuckle down

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'get to work' is the definition.
(I've seen this in another clue)

'joint wretched' is the wordplay.
'joint' becomes 'knuckle' (joint in the hand).
'wretched' becomes 'down' (I can't justify this - if you can you should give a lot more credence to this answer).
'knuckle'+'down'='KNUCKLE DOWN'

Can you help me to learn more?

(Other definitions for knuckle down that I've seen before include "Use some application" , "Apply oneself to hard work" , "Apply oneself seriously" , "Joint not working" , "Begin serious work" .)

I've seen this clue in The Telegraph.
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