Mercian king after whom a large earthwork is named (4)

I believe the answer is:

offa

(Other definitions for offa that I've seen before include "He created barrier" , "M arks of holiness borne by, eg, St Francis of Assisi" , "Dyke-building king of Mercia" , "Old dyke builder, d. 796" , "Mercian king, built dyke" .)

I've seen this clue in The Guardian.

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