Faith treats initial corrosion (5)
I believe the answer is:
trust
'faith' is the definition.
(I know that faith can be written as trust)
'treats initial corrosion' is the wordplay.
'initial' indicates taking the first letters.
'corrosion' becomes 'rust' (rust is a kind of corrosion).
The initial letter of 'treats' is 't'.
't'+'rust'='TRUST'
(Other definitions for trust that I've seen before include "Put one's faith in" , "Confident expectation" , "Faith - guardianship - after 10, the owners of the Guardian" , "responsibility" , "Have faith in, regard as reliable" .)