Twin (9)
I believe the answer is:
identical
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">twin</span>' is the definition.<br/ ><span class="explanation-format__explanation">(identical twin is a kind of twin)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for identical that I've seen before include "Impossible to tell apart" , "Selfsame" , "Similar" , "Like two peas in a pod" , "Duplicate" .)