Exclude a law that’s wrong (4,3)
I believe the answer is:
rule out
'exclude' is the definition.
(I've seen this before)
'a law that's wrong' is the wordplay.
'a law' becomes 'rule' (rule is a kind of law).
'that's wrong' becomes 'out' (eg a calculation might be 'out').
'rule'+'out'='RULE OUT'
(Other definitions for rule out that I've seen before include "'Eliminate from consideration (4,3)'" , "discount" , "Exclude as a possibility" , "Exclude from consideration" , "Dismiss from consideration" .)