Most jubilant (8)
I believe the answer is:
merriest
'most jubilant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for merriest that I've seen before include "most tipsy" , "Most intoxicated?" , "higher than the rest" , "Most jolly and high spirited" , "Jolliest" .)