knuckle down

knuckle down AND buckle down (to apply oneself earnestly to a task; focus; concentrate; commit to getting something done) — взяться за дело; серьезно отнестись к какому-л. делу; ≅ засучить рукава; ~ взяться за ум наконец (контекст)

Example 1: Tania used to party all the time, but lately she’s been trying to buckle down.

Example 2: I have to buckle down and study tonight. (EB)

see also
[взять быка за рога]
[roll up one's sleeves]
[засучить рукава]