The crossword solution for the term LEAD-IN TO SON OR SONG is SWAN, which is a four letter word. A SWAN is a large waterbird with a long neck and a graceful gliding motion. It is often used as a metaphor for beauty and grace in literature and music. In this context, it can be used to refer to the introduction of a son or song, which is typically a short musical phrase that sets the tone for the rest of the piece.