The term 'Instrument with some heft' is a crossword solution that can be filled in with the word 'TUBA' (4 letters). The TUBA is a large, low-pitched brass instrument that is used in many types of music, including classical, jazz, and marching bands. It is one of the most recognizable instruments in the brass family due to its size and sound. The TUBA is usually played with a mouthpiece and has valves that allow the player to change the pitch of the notes they are playing.