Tory reform of a system of weights (4)
I believe the answer is:
troy
'a system of weights' is the definition.
(troy is a kind of system of weights)
'tory reform' is the wordplay.
'reform' is an anagram indicator (letters in a new form).
'tory' is an anagram of 'TROY'.
'of' is the link.
(Other definitions for troy that I've seen before include "Besieged city which fell to Wooden Horse" , "Precious stone weight system" , "City impacted by long-term investment" , "Wooden horse siege city" , "Ancient city besieged by the Greeks" .)