Leader in a pact that's been arranged (7)
I believe the answer is:
captain
'leader' is the definition.
(captain is a kind of leader)
'in a pact that's been arranged' is the wordplay.
'that's been arranged' indicates an anagram.
'in'+'a'+'pact'='inapact'
'inapact' with letters rearranged gives 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Army rank" , "soldier" , "Naval commander" , "Head of a ship's crew" , "eg Hornblower" .)