Be rejected - that's bad (4)
I believe the answer is:
evil
'bad' is the definition.
(I know that evil is a type of bad)
'be rejected' is the wordplay.
'be' becomes 'live' (to be is to live or exist).
'rejected' says the letters should be written in reverse.
'live' back-to-front is 'EVIL'.
'that's' is the link.
(Other definitions for evil that I've seen before include "Morally objectionable behavior" , "Villainy" , "Morally reprehensible" , "Immorality" , "Excellent" .)