Faith to decay after time (5)
I believe the answer is:
trust
'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.
(Other definitions for trust that I've seen before include "bank on" , "Firm belief (in a thing)" , "Have faith in, regard as reliable" , "Faith; care" , "commercial credit" .)