Supporter to begin accepting terrible law (8)
I believe the answer is:
stalwart
'supporter' is the definition.
(stalwart is a kind of supporter)
'begin accepting terrible law' is the wordplay.
'begin' becomes 'start' (synonyms).
'accepting' means one lot of letters goes inside another.
'terrible' indicates an anagram.
'law' is an anagram of 'lwa'.
'start' placed around 'lwa' is 'STALWART'.
'to' is the link.
(Other definitions for stalwart that I've seen before include "Normally reliable" , "Determined" , "Steadfast" , "stout" , "Tart laws could be strong and sturdy" .)