A tough problem is a difficult challenge that requires a lot of thought and effort to solve. The crossword solutions for this term include 'STINKER' (7 letters), 'not' (3 letters), 'DILEMMA' (7 letters), 'POSER' (5 letters), and 'KNOT' (4 letters). All of these words can be used to describe a tough problem, as they all imply a difficult situation that is hard to solve. A STINKER is an especially difficult problem, while a DILEMMA is a situation with two equally undesirable options. A POSER is a question or problem that is difficult to answer, and a KNOT is a complex entanglement that is hard to unravel.