Trusted to contain oxidation (4)
I believe the answer is:
rust
'oxidation' is the definition.
(I know that rust is a type of oxidation)
'trusted to contain' is the wordplay.
'to contain' indicates the answer is hidden within the clue (words from the clue contain the answer).
'RUST' is hidden in the letters of 'trusted'.
(Other definitions for rust that I've seen before include "Corrode" , "Reddish-brown coating caused by oxidation" , "Oxidised iron" , "Reddish-brown oxide coating" , "deteriorate" .)