The term 'HEFTY INSTRUMENT' can be solved by the four letter word 'TUBA'. The TUBA is a large, conical-bore brass instrument that is the lowest-pitched instrument in the brass family. It is one of the most recent additions to the modern symphony orchestra, first appearing in the mid-19th century, and is used in both classical and jazz ensembles. The TUBA is known for its deep, rich sound and is often used to provide a strong bass line in musical compositions.