A petty officer, say, reveals defector (8)
I believe the answer is:
apostate
'defector' is the definition.
(I've seen this before)
'a petty officer say' is the wordplay.
'petty officer' becomes 'po' (naval abbreviation).
'say' becomes 'state' (synonyms).
'a'+'po'+'state'='APOSTATE'
'reveals' is the link.
(Other definitions for apostate that I've seen before include "One going to other side" , "Person who deserts their religion" , "One who renounces a belief" , "heretic" , "Renegade" .)