From Longman Dictionary of Contemporary Englisha hard/tough nut to cracka hard/tough nut to cracka difficult problem or situation Celtic have lost only once this season and will be a tough nut to crack. → nut
Examples from the Corpus
a hard/tough nut to crack• West Ham will be a tough nut to crack especially with big Lee in good form at the moment.• Daytime television is a tough nut to crack. New shows have to be good enough to beat the old favorites.• Already highly successful in popular music, dance and commercial television, blacks have found the movies a tougher nut to crack.• Beverley was a tougher nut to crack. 