Cruel ruler (6)
I believe the answer is:
tyrant
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cruel ruler</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 tyrant that I've seen before include "Despotic ruler" , "Martinet" , "bully - with 17d" , "Person dictating" , "Harsh and absolute ruler" .)