Account supported by a clergyman should be reliable (8)
I believe the answer is:
accurate
'reliable' is the definition.
('accurate' can be similar in meaning to 'reliable')
'account supported by a clergyman' is the wordplay.
'account' becomes 'ac'.
'supported by' is a charade indicator (letters next to each other) (letters go below - in down clue).
'a clergyman' becomes 'curate' (curate is a kind of clergyman).
'ac'+'curate'='ACCURATE'
'should be' acts as a link.
(Other definitions for accurate that I've seen before include "Meticulous" , "Correct, right" , "faithful" , "Bang on" , "Spot on" .)