Crossword Genius

One who's lost faith in a petty officer, say (8)

Ross

I believe the answer is:

apostate

'one who's lost faith' is the definition.

'a petty officer say' is the wordplay.
'petty officer' becomes 'po' (naval abbreviation).
'say' becomes 'state' (synonyms).
'a'+'po'+'state'='APOSTATE'

'in' acts as a link.

(Other definitions for apostate that I've seen before include "One no longer believing", "Deserte", "Deserter", "Person who has renounced former belief", "Dissenter".)

I've seen this clue in The Telegraph.