Wiktionary
tough nut to crack
alt. 1 (&lit tough nut crack English) 2 (context idiomatic English) A problem that is challenging to solve. 3 (context idiomatic by extension English) A situation, person, group, etc. which is difficult to deal with. 4 (context idiomatic English) A place, opportunity, etc. to which it is difficult to gain entry. 5 An amount that is difficult to finance. n. 1 (&lit tough nut crack English) 2 (context idiomatic English) A problem that is challenging to solve. 3 (context idiomatic by extension English) A situation, person, group, etc. which is difficult to deal with. 4 (context idiomatic English) A place, opportunity, etc. to which it is difficult to gain entry. 5 An amount that is difficult to finance.