A harsh order issued by authority (6)
I believe the answer is:
diktat
'a 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 "Official order" , "Order allowing no opposition" , "Undemocratic decree" , "Harsh order issued by authority" , "Harsh order given by someone with authority" .)