Mercian king (4)
I believe the answer is:
offa
'mercian king' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for offa that I've seen before include "Old dyke-builder" , "Mercian dykemaker" , "Mercian king, built dyke" , "Eighth-century king of Mercia" , "Eighth century king of Mercia" .)