The crossword solution for the term 'IIGHT, HAPPY TUNE' is 'LILT', which is a four-letter word. A LILT is a light, cheerful tune or melody, often with a skipping or hopping rhythm. It is commonly used in folk music and traditional Irish music, as well as in some popular music genres such as jazz and rock. The term can also refer to a particular type of vocal delivery, where the singer's voice rises and falls in a melodic pattern.