Winter Clusters gravitate toward honey stores in an 'upward' direction. Bees clustered in the top box indicates that stores have likely been exhausted, although it can be amazing how much honey sometimes is remaining below and/or on the sides. Being in the top, they 'may' be doomed already depending on how much winter is left. Keep an eye on them and KEEP FEEDING until Spring arrives, either with dry sugar, candy or honey. Good Luck!
NOTE; other than quick inspections to determine and replenish stores, no other frame manipulations should be performed and the winter cluster should NOT be disturbed other than to see where it is….if possible. If you can't 'see' the cluster but there are obvious signs of bees and honey inside, generally all will be OK. Just leave them Beeee. :) Winter inspections should be avoided, EXCEPT to determine remaining stores.