Decreed it should be under the dominion of (5)
I believe the answer is:
ruled
'decreed' is the definition.
('rule' can be a synonym of 'decree')
'under the dominion of' is the wordplay.
I cannot quite see how this works, but
'dominion' could be 'rule' (synonyms) and 'rule' is found in the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'it should be' acts as a link.
Can you help me to learn more?
(Other definitions for ruled that I've seen before include "Governed or commanded" , "Governed - with lines" , "Made authoritative statement" , "Governed, used one's authority" , "with several lines" .)