Bringer of law, measure that performer must follow (7)
I believe the answer is:
enactor
'bringer of law' is the definition.
(someone enacts a law)
'measure that performer must follow' is the wordplay.
'measure' becomes 'en' (en is a kind of measure).
'that performer' becomes 'actor' (actor is a kind of performer).
'must follow' says to put letters next to each other.
'en'+'actor'='ENACTOR'
(Other definitions for enactor that I've seen before include "lawman!" , "Law-maker" .)