Invincible (10)
I believe the answer is:
unbeatable
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">invincible</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that invincible can be written as unbeatable)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for unbeatable that I've seen before include "Invincible" , "best" .)