Loyalist beginning to pen criminal law (8)
I believe the answer is:
stalwart
'loyalist' is the definition.
('stalwart' can be a synonym of 'loyalist')
'beginning to pen criminal law' is the wordplay.
'beginning' becomes 'start' ('start' can be a synonym of 'beginning').
'to pen' is an insertion indicator.
'criminal' indicates an anagram.
'law' anagrammed gives 'lwa'.
'start' going around 'lwa' is 'STALWART'.
(Other definitions for stalwart that I've seen before include "Loyalist" , "Loyal supporter" , "stout" , "Resolute and firm" , "Committed" .)