Editor about to get set of principles laid down (7)
I believe the answer is:
decreed
'laid down' is the definition.
(to decree is to lay down a law)
'editor about to get set of principles' is the wordplay.
'editor' becomes 'ed'.
'about' says the letters should be written backwards.
'to get' says to put letters next to each other.
'set of principles' becomes 'creed' (set of religious beliefs).
'ed' reversed gives 'de'.
'de'+'creed'='DECREED'
(Other definitions for decreed that I've seen before include "Ordained" , "Officially ordered" .)