Most jubilant (8)
I believe the answer is:
merriest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most jubilant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for merriest that I've seen before include "most tipsy" , "Most intoxicated?" , "higher than the rest" , "Most jolly and high spirited" , "Jolliest" .)