Not sparkling
I believe the answer is:
still
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not sparkling</span>' is the definition.<br/ ><span class="explanation-format__explanation">('still' is the opposite to 'sparkling')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for still that I've seen before include "Yet - hushed" , "sort of life" , "not like champagne" , "Undisturbed < , yet" , "subdued" .)