Be up for wickedness (4)
I believe the answer is:
evil
'wickedness' is the definition.
('evil' can be a synonym of 'wickedness')
'be up' is the wordplay.
'be' becomes 'live' (living is a kind of being).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'live' written backwards gives 'EVIL'.
'for' is the link.
(Other definitions for evil that I've seen before include "Wicked - sin" , "Wicked - social problem" , "Badness, depravity" , "Great wickedness" , "Maleficent" .)