Entrusted with oxidation (4)
I believe the answer is:
rust
'oxidation' is the definition.
(rust is a kind of oxidation)
'entrusted with' is the wordplay.
'with' indicates a hidden word.
'RUST' is hidden in the letters of '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" .)