Wickedness, as it happens to return (4)
I believe the answer is:
evil
'wickedness' is the definition.
('evil' can be a synonym of 'wickedness')
'as it happens to return' is the wordplay.
'as it happens' becomes 'live' (as in live television).
'to return' says the letters should be written backwards.
'live' back-to-front is 'EVIL'.
(Other definitions for evil that I've seen before include "source of harm" , "heinous" , "Malignant" , "Badness, depravity" , "Excellent" .)