Key legislation's defect (4)
I believe the answer is:
flaw
'defect' is the definition.
('flaw' can be a synonym of 'defect')
'key legislation's' is the wordplay.
'key' becomes 'f' (I've seen this in other clues).
'legislation' becomes 'law' (I've seen this before**).
'f'+'law'='FLAW'
(Other definitions for flaw that I've seen before include "sudden rush" , "Mark, blemish" , "Weakness" , "Error in theory or plan" , "(Make) defect(ive)" .)