knuckle
verb/ˈnʌkl/ 
PHRASAL VERBS knuckle down (to something)
(informal) to begin to work hard at something synonym get down toI'm going to have to knuckle down to some serious study.knuckle downknuckle down toknuckle under (to someone/something)
(informal) to accept someone else's authorityknuckle underknuckle under to