Sailor — deserter heading west (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'deserter heading west' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'heading west' says the letters should be written backwards.
'rat' backwards is 'TAR'.
(Other definitions for tar that I've seen before include "Informal sailor or road surfacing material" , "Naval man" , "prepare for feathers?" , "Pitch; sailor" , "Mariner, old salt" .)