Impervious, unyielding (9)
I believe the answer is:
resistant
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">impervious</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">unyielding</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for resistant that I've seen before include "unaffected" , "Rebel" , "Withstanding adverse conditions" , "Not easily convinced" , "Impervious (to corrosion, heat, etc)" .)