King of Mercia from 757 associated with an earthwork roughly following some of the current border between England and Wales (4)

I believe the answer is:

offa

(Other definitions for offa that I've seen before include "Earthwork maker" , "One-time king" , "Eighth-century king of Mercia" , "Mercian dykemaker" , "Mercian leader" .)

I've seen this clue in The Independent.

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