Historic leader disengaged from area (4)
I believe the answer is:
offa
'historic leader' is the definition.
(King of Mercia)
'disengaged from area' is the wordplay.
'disengaged from' becomes 'off' (as in going 'off' something or someone).
'area' becomes 'a' (maths abbreviation).
'off'+'a'='OFFA'
(Other definitions for offa that I've seen before include "Entrenched person" , "King of Mercia, who constructed earthwork on his Welsh border" , "Eighth-century king of Mercia" , "Organiser of defence" , "Mercia's dyke builder" .)