Old kingdom of Crimea devastated (6)
I believe the answer is:
mercia
'old kingdom' is the definition.
(Anglo-Saxon kingdom)
'crimea devastated' is the wordplay.
'devastated' indicates anagramming the letters (devastate literally means destroy or lay waste to).
'crimea' is an anagram of 'MERCIA'.
'of' is the link.
(Other definitions for mercia that I've seen before include "share of Albion?" , "part of old England" , "Former kingdom of central England" , "Former English kingdom" , "Ancient midland kingdom" .)