Order reduction when leaving (6)
I believe the answer is:
decree
'order' is the definition.
('decree' can be a synonym of 'order')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for decree that I've seen before include "Judgment of a court" , "Ordinance" , "Ordain" , "Official edict" , "Edict; court order" .)