Mercian king, AD757-796 (4)

I believe the answer is:

offa

(Other definitions for offa that I've seen before include "8th-century Mercian king" , "Old English dyke builder" , "Mercian leader" , "King of Mercia, who constructed earthwork on his Welsh border" , "King of Mercia, who may have built a dyke" .)

I've seen this clue in The Guardian.

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