Turning tail, sailor is a traitor (3)
I believe the answer is:
rat
'a traitor' is the definition.
(rat can mean a treacherous person)
'turning tail sailor' is the wordplay.
'turning tail' says the letters should be written backwards.
'sailor' becomes 'tar' (term for a sailor).
'tar' reversed gives 'RAT'.
'is' acts as a link.
(Other definitions for rat that I've seen before include "Betrayer" , "One deserting sinking ship" , "Domestic rodent" , "Despicable person" , "Contemptible person" .)