King of Mercia from 757 associated with an earthwork between England and Wales (4)

I believe the answer is:

offa

(Other definitions for offa that I've seen before include "Old Mercia king" , "One-time ruler" , "Mercian king, built dyke" , "Mercian king, dyke builder" , "Dyke-building king of Mercia" .)

I've seen this clue in The Independent.

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