Strong supporter of institute introducing new law (8)
I believe the answer is:
stalwart
'strong supporter' is the definition.
(I've seen this before)
'institute introducing new law' is the wordplay.
'institute' becomes 'start' (to institute something is to begin or start it).
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'new' indicates an anagram.
'law' with letters rearranged gives 'alw'.
'start' enclosing 'alw' is 'STALWART'.
'of' acts as a link.
(Other definitions for stalwart that I've seen before include "Committed" , "Steadfast" , "Staunch; loyal" , "Normally reliable" , "Loyalist" .)