Show surprise about law-breaking supporter (8)
I believe the answer is:
stalwart
'supporter' is the definition.
(stalwart is a kind of supporter)
'show surprise about law-breaking' is the wordplay.
'show surprise' becomes 'start'.
'about' is an anagram indicator.
'breaking' indicates putting letters inside.
'law' with letters rearranged gives 'lwa'.
'start' placed around 'lwa' is 'STALWART'.
(Other definitions for stalwart that I've seen before include "Brick" , "Tart laws could be strong and sturdy" , "Steadfast" , "Loyal, reliable and hardworking" , "Reliable; sturdy" .)