Deserter heading north is a sailor (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'deserter heading north' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'heading north' is a reversal indicator.
'rat' in reverse letter order is 'TAR'.
'is a' is the link.
(Other definitions for tar that I've seen before include "Familiar sailor - what a rat" , "Pitch; sailor" , "Sticky road material or informal sailor" , "One sails on sea" , "Cigarette substance" .)