Unchangeable (9)
I believe the answer is:
immutable
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unchangeable</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for immutable that I've seen before include "Not able to be changed" , "Changeless" .)