Traitor making arrangement to cede fort (8)
I believe the answer is:
defector
'traitor' is the definition.
(thesaurus)
'arrangement to cede fort' is the wordplay.
'arrangement' is an anagram indicator.
'cede'+'fort'='cedefort'
'cedefort' is an anagram of 'DEFECTOR'.
'making' acts as a link.
(Other definitions for defector that I've seen before include "E.g. Philby" , "One joining the opposition" , "One who goes over to the enemy" , "Turncoat" , "subject of court martial?" .)