The term 'Tough One' can be used to describe a variety of difficult tasks. It can be represented by the crossword solutions 'STUMPER' (7 letters), 'POSER' (5 letters), 'TEASER' (6 letters), and 'LABORIOUS' (9 letters). A STUMPER is a difficult question or problem that is hard to answer or solve. A POSER is a difficult problem or question that requires thought and effort to solve. A TEASER is a difficult problem or puzzle that is designed to challenge one's mental abilities. Finally, LABORIOUS is an adjective used to describe something that requires a lot of effort and hard work.