Robust law changed in shock (8)
I believe the answer is:
stalwart
'robust' is the definition.
(similar in meaning)
'law changed in shock' is the wordplay.
'changed' indicates anagramming the letters.
'in' is an insertion indicator.
'shock' becomes 'start' (to give someone a start is to shock them).
'law' with letters rearranged gives 'alw'.
'alw' inserted into 'start' is 'STALWART'.
(Other definitions for stalwart that I've seen before include "Valiant" , "Walt's rat is strong and brave" , "One you can rely on" , "Committed" , "Staunch supporter" .)