Act as opponent regarding law passed (5)
I believe the answer is:
react
'act as opponent' is the definition.
(I've seen this before)
'regarding law passed' is the wordplay.
'regarding' becomes 're' ('re' means regarding or about).
'law passed' becomes 'act' (I've seen this before).
're'+'act'='REACT'
(Other definitions for react that I've seen before include "Change chemically" , "Behave in response" , "Retaliate" , "What a chemical might do" , "Respond to a stimulus" .)