hard nut to crack

noun4

1. •
   A problem that is challenging to solve.

   • He saw the dilemma. . . . It was a hard nut to crack. He could see no way out of it. — 1893
2. •
   A situation, person, group, etc. which is difficult to overcome or deal with.

   • The coast lad found the veteran Mike McTigue a hard nut to crack and judging from the look on the Californian's face when the final bell sounded, he was mighty happy that the fight was over. — 1928
3. •
   A place, opportunity, etc. to which it is difficult to gain entry.

   • Durango, however, may be a hard nut to crack, as it is strong strategically and is reported guarded by 4000 rebels. — 1929
4. •
   An amount that is difficult to finance.