Naval officer in a condition to become deserter (8)
I believe the answer is:
apostate
'deserter' is the definition.
(I know that deserter can be written as apostate)
'naval officer in a condition' is the wordplay.
'naval officer' becomes 'po' (abbreviation for petty officer).
'in' indicates putting letters inside.
'condition' becomes 'state' (the state something is in).
'a'+'state'='astate'
'po' going into 'astate' is 'APOSTATE'.
'to become' is the link.
(Other definitions for apostate that I've seen before include "Person abandoning their religion" , "Religious rebel" , "Julian was such" , "One renouncing a belief" , "Renegade" .)