The riches derived from the law (6)
I believe the answer is:
wealth
'the riches' is the definition.
(synonyms)
'derived from the law' is the wordplay.
'derived from' is an anagram indicator.
'the'+'law'='thelaw'
'thelaw' anagrammed gives 'WEALTH'.
(Other definitions for wealth that I've seen before include "was common under Cromwell" , "'Riches, affluence (6)'" , "Mammon" , "Assets" , "Prosperity" .)