Faithful, a Catholic clergyman (8)
I believe the answer is:
accurate
'faithful' is the definition.
('accurate' can be similar in meaning to 'faithful')
'a catholic clergyman' is the wordplay.
'catholic' becomes 'c' (abbreviation as in RC for Roman Catholic).
'clergyman' becomes 'curate' (curate is a kind of clergyman).
'a'+'c'+'curate'='ACCURATE'
(Other definitions for accurate that I've seen before include "Precise, correct" , "Care a cut is exact" , "True!" , "Correct, right" , "Spot-on" .)