O compound (5)
I believe the answer is:
oxide
'o compound' is the definition.
(I know that oxide is a type of chemical compound)
This is all the clue.
(Other definitions for oxide that I've seen before include "Eg, rust" , "Compound such as rust" , "Rust is iron -" , "Compound of two elements, one with atomic number 8" , "Rust, for instance" .)