Harsh order issued by authority (6)
I believe the answer is:
diktat
'harsh order issued by authority' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for diktat that I've seen before include "Categorical decree" , "Harsh order given by someone with authority" , "An order that allows no opposition" , "Harsh authoritative order" , "Harsh order or decree" .)