Respond with regard to law (5)
I believe the answer is:
react
'respond' is the definition.
(synonyms)
'with regard to law' is the wordplay.
'with regard' becomes 're'.
'to law' becomes 'act' (I've seen this before).
're'+'act'='REACT'
(Other definitions for react that I've seen before include "Retaliate" , "What a chemical might do" , "Trace (anag)" , "Perform again" , "Change chemically" .)