Bad user, for certain (4)
I believe the answer is:
sure
'certain' is the definition.
(synonyms)
'bad user' is the wordplay.
'bad' indicates an anagram.
'user' is an anagram of 'SURE'.
'for' acts as a link.
(Other definitions for sure that I've seen before include "of course" , "certainly in the USA" , "Undoubtedly true" , "Feeling no doubt" , "Convinced" .)