Hilariously funny (7)
I believe the answer is:
riotous
'hilariously funny' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for riotous that I've seen before include "Behaving wildly" , "wild and unruly" , "Rowdy" , "Of mob, disorderly and unruly" , "Wild, uncontrolled" .)