foot
The 'missing word' is wrap.
Wrap
soot
Duel and fuel are two rhyming words.
The word you are looking for is "mass."
snap clap crap trap slap and more
Rock.
kink
Shoe Shot Soot Coot Coat
Ashy is a four-letter adjective that means covered in ash or ash-colored. Soot, dust, or slag are four-letter nouns meaning ash.
The answer is soot. If you remove the letter "t" from "soot," you get "soo," which is not a valid word.
sooty