The term 'impish one' can be defined by four different crossword solutions: BRAT (4 letters), pixy (4 letters), RASCAL (6 letters), ELF (3 letters), and SCAMP (5 letters). A BRAT is a mischievous child who behaves badly. A pixy is a mischievous fairy or sprite. A RASCAL is someone who behaves in a mischievous or naughty way. An ELF is a supernatural creature with magical powers, often associated with mischief. Finally, a SCAMP is someone who behaves in a mischievous or naughty way. All of these words can be used to describe an impish one, which is someone who behaves in a mischievous or naughty way.