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