Entrusted with oxidation (4)
I believe the answer is:
rust
'oxidation' is the definition.
(I know that rust is a type of oxidation)
'entrusted with' is the wordplay.
'with' says the answer is hidden in the clue.
'RUST' is hidden within 'entrusted'.
(Other definitions for rust that I've seen before include "Corrode" , "Oxidisation" , "Red or brown oxide coating on iron or steel" , "Reddish-brown oxide coating" , "Reddish-brown coating formed on metal by moisture" .)