King of Mercia (4)
I believe the answer is:
offa
'king of mercia' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for offa that I've seen before include "Eighth-century king of Mercia" , "8th-century Mercian king" , "top man in Mercia" , "man throwing up defence" , "King of Mercia, who may have built a dyke" .)