Tsarist edict (5)
I believe the answer is:
ukase
'tsarist edict' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ukase that I've seen before include "Old Russian edict" , "Official or arbitrary decree" , "Decree (originally from the Tsar)" , "Legal order" , "Tsarist decree; sea UK (anag.)" .)