Lime or rust (5)
I believe the answer is:
oxide
'lime or rust' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oxide that I've seen before include "Rust is iron -" , "Rust, for instance" , "Eg, rust" , "Compound of two elements, one with atomic number 8" , "Compound such as rust" .)