Faith to decay after time (5)
I believe the answer is:
Here is my best explanation:
'faith' is the definition.
('trust' can be a synonym of 'faith')
'decay after time' is the wordplay.
'decay' becomes 'rust' (rusting is a kind of decaying**).
'after' says to put letters next to each other.
'time' becomes 't'.
'rust' after 't' is 'TRUST'.
'to' is the link.