Apply oneself diligently (7,4)

I believe the answer is:

knuckle down

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

I've seen this clue in The Mirror.

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.