Compound (5)
I believe the answer is:
oxide
'compound' is the definition.
(I know that oxide is a type of compound)
This is the entire clue.
(Other definitions for oxide that I've seen before include "Compound of oxygen with another element" , "Compound of oxygen and one other element" , "Oxygenous compound" , "Rust is iron -" , "Compound of two elements, one with atomic number 8" .)