knuckle down to (do) (something)
knuckle down to (do) (something)
To begin to undertake something in a determined and focused matter. The whole team will need to knuckle down to this project if we want to get it done by April. I have to give her credit—she really knuckled down to finish her degree.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.