The term FLUE FILTH is composed of two words, 'FLUE' and 'FILTH'. The first word, 'FLUE', is a noun that refers to the duct or passage through which smoke or gas is emitted from a chimney. The second word, 'FILTH', is a noun that refers to dirt, grime, or refuse. Together, the term FLUE FILTH can refer to the SOOT and other debris that accumulates in a FLUE due to the burning of fuel. The crossword solution for this term is SOOT, which is a fine black powdery substance composed of carbon particles that is produced by the incomplete combustion of fuel.