The term 'Tough Nut to Crack' is an idiom used to describe a difficult problem or situation. The crossword solutions for this term are ACORN (5 letters), PECAN (5 letters), ENIGMA (6 letters), and POSER (5 letters). Acorns and pecans are both types of nuts that have hard shells, making them difficult to crack open. An ENIGMA is a puzzle or riddle that is difficult to solve, and a POSER is a difficult question or problem. All of these words can be used to describe a tough nut to crack.