Firm breaking law in the beginning (8)
I believe the answer is:
stalwart
'firm' is the definition.
(I've seen this in another clue)
'breaking law in the beginning' is the wordplay.
'breaking' indicates anagramming the letters.
'in' indicates putting letters inside.
'the beginning' becomes 'start' (start is a kind of beginning).
'law' with letters rearranged gives 'alw'.
'alw' inserted inside 'start' is 'STALWART'.
(Other definitions for stalwart that I've seen before include "husky" , "Dependable figure" , "Determined" , "Staunchly faithful" , "Brick" .)