Unreactive (5)
I believe the answer is:
inert
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unreactive</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for inert that I've seen before include "Devoid of motion or change" , "Chem. non-reactive" , "Motionless, still" , "like neon" , "Chemically unreactive" .)