Deserter turned back on sailor (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'deserter turned back' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'turned back' says the letters should be written in reverse.
'rat' in reverse letter order is 'TAR'.
'on' is the link.
(Other definitions for tar that I've seen before include "Flammable liquid; sailor" , "Art (anag.)" , "Informal name for sailor" , "Viscous liquid; sailor" , "Familiar sailor - what a rat" .)