Operate by decree (3)
I believe the answer is:
act
I believe this clue is a double definition.
'operate' is the first definition.
(thesaurus)
'decree' is the second definition.
(I've seen this in other clues)
'by' acts as a link.
(Other definitions for act that I've seen before include "Do something, in a play?" , "Take steps" , "Exploit" , "Perform - part of a play" , "law passed" .)