What the radio repairer knuckles down to getting? (4,2,4)

I believe the answer is:

sets to work

I'm a little stuck... Click here to teach me more about this clue!

'what the radio repairer knuckles' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'down to getting?' is the wordplay.
I cannot really understand how this works, but
'down' could be 'set' (similar in meaning) and 'set' is present in the answer.
'to' is within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.