Law, say, is changed for good (6)
I believe the answer is:
always
'for good' is the definition.
(I've seen this before)
'law say is changed' is the wordplay.
'is changed' indicates anagramming the letters.
'law'+'say'='lawsay'
'lawsay' is an anagram of 'ALWAYS'.
(Other definitions for always that I've seen before include "Forever, without interruption" , "On all occasions" , "In perpetuity" , "For good" , "Invariably" .)