Upsetting gag, it ain't funny (9)
I believe the answer is:
agitating
'upsetting' is the definition.
(agitating is a kind of upseting)
'gag it ain't funny' is the wordplay.
'funny' indicates anagramming the letters.
'gag'+'it'+'aint'='gagitaint'
'gagitaint' with letters rearranged gives 'AGITATING'.
(Other definitions for agitating that I've seen before include "Disturbing" , "Shaking" , "stirring things up" .)