Originally filed under national merriment? (3)
I believe the answer is:
fun
'merriment?' is the definition.
('fun' can be a synonym of 'merriment')
'originally filed under national' is the wordplay.
'originally' says to take the initial letters.
The initial letters of 'filed under national' is 'FUN'.
(Other definitions for fun that I've seen before include "Entertaining" , "Jollification" , "Enjoyment" , "Playful amusement" , "Light-hearted amusement, jollity" .)