The term 'HEFTY HORN' is a combination of two words, 'HEFTY' and 'HORN'. HEFTY is an adjective meaning large or bulky, while HORN is a noun referring to a brass wind instrument. The crossword solution for this term is 'TUBA', a four-letter word. A TUBA is a large, low-pitched brass instrument that is often used in orchestras, marching bands, and jazz ensembles. It is the largest and lowest-pitched brass instrument in the modern symphony orchestra.