Old victory secured by sovereign territory (8)
I believe the answer is:
province
'territory' is the definition.
(I've seen this before)
'old victory secured by sovereign' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'victory' becomes 'v'.
'secured by' is an insertion indicator (I've seen this in other clues).
'sovereign' becomes 'prince' (I've seen this before**).
'o'+'v'='ov'
'ov' inserted into 'prince' is 'PROVINCE'.
(Other definitions for province that I've seen before include "Territorial division" , "Administrative region" , "Canterbury, for example" , "Sphere of action or speciality" , "Administrative district of country" .)