Western county in former kingdom (6)
I believe the answer is:
wessex
'former kingdom' is the definition.
(historical English kingdom)
'western county' is the wordplay.
'western' becomes 'w'.
'county' becomes 'essex' (I've seen this before).
'w'+'essex'='WESSEX'
'in' acts as a link.
(Other definitions for wessex that I've seen before include "South-west counties of Hardy's novels" , "Alfred the Great's realm" , "Hardy's West Country" , "ancient region" , "Prince Edward, earl of ..." .)