Highly unusual (13)
I believe the answer is:
extraordinary
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">highly unusual</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for extraordinary that I've seen before include "Remarkable - abnormal" , "Phenomenal" , "Highly unusual" , "Exceptional" .)