Showing a certain incorruptibility unusual for Proust (9)
I believe the answer is:
rustproof
'showing a certain incorruptibility' is the definition.
(unable to be rusted)
'unusual for proust' is the wordplay.
'unusual' is an anagram indicator.
'for'+'proust'='forproust'
'forproust' is an anagram of 'RUSTPROOF'.
(Other definitions for rustproof that I've seen before include "(Of iron) resistant to oxidisation" , "Impervious to irony tainting" , "(Of iron) not susceptible to corrosion" .)