Sailor returns as traitor (3)
I believe the answer is:
rat
'traitor' is the definition.
(rat can mean a traitor)
'sailor returns' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'returns' says the letters should be written in reverse.
'tar' back-to-front is 'RAT'.
'as' acts as a link.
(Other definitions for rat that I've seen before include "one escaping doomed vessel?" , "Treacherous rodent" , "Desert - rodent" , "Betray like a rodent" , "Renegade" .)