The term 'sing smoothly' can be summarized by the crossword solution 'CROON, ' which is a five-letter word. Crooning is a style of singing that is characterized by a smooth, gentle, and melodic sound. It is often associated with jazz and blues music, as well as popular music from the early 20th century. Crooning is also known for its use of vibrato, a technique used to create a wavering effect in the singer's voice.