No longer fizzy (4)
I believe the answer is:
flat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">no longer fizzy</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for flat that I've seen before include "Dull; set of rooms" , "like some races" , "Featureless" , "Punctured tyre" , "Dull; apartment" .)