Most extreme (6)
I believe the answer is:
utmost
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most extreme</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 utmost that I've seen before include "The greatest possible degree" , "Finest" , "Maximum effort" , "At furthest limit" , "(To the) greatest possible extent" .)