Annul a law about a toll (6)
I believe the answer is:
repeal
'annul a law' is the definition.
(I've seen this before)
'about a toll' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a toll' becomes 'peal' (I've seen this in another clue).
're'+'peal'='REPEAL'
(Other definitions for repeal that I've seen before include "Annul an act of parliament" , "Annulment" , "Of law, rescind or cancel" , "Rescind or withdraw a law" , "reversal of act" .)