Of which there is only one (6)
I believe the answer is:
unique
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of which there is only one</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 unique that I've seen before include "Singular, one of a kind" , "One-off" , "incomparable" , "Singular, different from others" , "Single" .)