Ruling that restricts a chemical substance (7)
I believe the answer is:
reagent
'chemical substance' is the definition.
(I've seen this before)
'ruling that restricts a' is the wordplay.
'ruling' becomes 'regent' (I've seen this before).
'that restricts' is an insertion indicator.
'regent' placed around 'a' is 'REAGENT'.
(Other definitions for reagent that I've seen before include "Reacting substance" , "Substance used in chemical analysis" , "Chemical substance used in tests" , "Chemically-active substance" , "Get near (anag) - substance used in chemistry" .)