Bad upset, as it happens (4)
I believe the answer is:
live
'as it happens' is the definition.
(as in live television)
'bad upset' is the wordplay.
'bad' becomes 'evil' ('evil' is associated in meaning with 'bad').
'upset' says the letters should be written in reverse.
'evil' backwards is 'LIVE'.
(Other definitions for live that I've seen before include "Having a shocking potential" , "Present in person, not recorded" , "Eg, from Kingston" , "Dwell" , "A . . . . wire is a highly energetic person" .)