To veto old law is crazy (8)
I believe the answer is:
disallow
'to veto' is the definition.
('disallow' can be a synonym of 'veto')
'old law is crazy' is the wordplay.
'crazy' indicates anagramming the letters.
'old'+'law'+'is'='oldlawis'
'oldlawis' anagrammed gives 'DISALLOW'.
(Other definitions for disallow that I've seen before include "Reject" , "Rule out" , "Prohibit, veto" , "Prevent" , "Overrule" .)