King of Mercia (4)
I believe the answer is:
offa
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">king of mercia</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for offa that I've seen before include "Eighth-century king of Mercia" , "8th-century Mercian king" , "top man in Mercia" , "man throwing up defence" , "King of Mercia, who may have built a dyke" .)