Most uncommon (6)
I believe the answer is:
rarest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most uncommon</span>' is the definition.<br/ ><span class="explanation-format__explanation">('rare' can be similar in meaning to 'uncommon')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rarest that I've seen before include "Least common, most unusual" , "most excellent" , "Most scarce or least common" , "fine to maximum" , "Most scarce or infrequent" .)