Following legislation is a mistake (4)
I believe the answer is:
flaw
'a mistake' is the definition.
(thesaurus)
'following legislation' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'legislation' becomes 'law' (I've seen this before).
'f'+'law'='FLAW'
'is' acts as a link.
(Other definitions for flaw that I've seen before include "(Make) defect(ive)" , "Defect, imperfection" , "Error in theory or plan" , "failing" , "sudden rush" .)