One occupying a stronghold in an old kingdom (7)
I believe the answer is:
castile
'an old kingdom' is the definition.
(I've seen this before)
'one occupying a stronghold' is the wordplay.
'one' becomes 'i' (Roman numeral).
'occupying' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'a stronghold' becomes 'castle' (I've seen this before).
'i' put within 'castle' is 'CASTILE'.
'in' acts as a link.
(Other definitions for castile that I've seen before include "Old Spanish kingdom" , "Elastic region in central Spain" , "Elastic (anag) -- region of Spain" , "former Spanish kingdom" .)