Rule as a monarch (5)
I believe the answer is:
reign
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rule as a monarch</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 reign that I've seen before include "Be champion" , "Rule over in Niger" , "Sit on a throne" , "Period during which someone is dominant" , "Period of the rule of a sovereign" .)