Not better with sore bum (5)
I believe the answer is:
worse
'not better' is the definition.
('worse' is the opposite to 'better')
'with sore bum' is the wordplay.
'with' becomes 'w' (abbreviation).
'bum' indicates an anagram (bum can mean faulty or wrong).
'sore' with letters rearranged gives 'orse'.
'w'+'orse'='WORSE'
(Other definitions for worse that I've seen before include "Less good" , "Bad to a greater degree" , "more unpleasant" , "Opposite of better" , "less moral?" .)