Most collectible (6)
I believe the answer is:
rarest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most collectible</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rarest that I've seen before include "fine to maximum" , "least done" , "most excellent" , "Most scarce or uncommon" , "Sartre (anag) -- most uncommon" .)