Entrusted with corrosion (4)
I believe the answer is:
rust
'corrosion' is the definition.
(rust is a kind of corrosion)
'entrusted with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'RUST' is hidden in the letters of 'entrusted'.
(Other definitions for rust that I've seen before include "Reddish-brown coating formed on metal by moisture" , "Reddish-brown oxide coating" , "Reddish-brown colour" , "Fungal plant disease" , "Corrode" .)