Uncommon object (6)
I believe the answer is:
rarity
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">uncommon object</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 rarity that I've seen before include "Curio (1)" , "Thinness" , "Unusual specimen" , "Something unusual, perhaps collectible" , "something quite unusual" .)