Decree establishing four nations as one ultimately (5)
I believe the answer is:
ukase
'decree' is the definition.
(I know that ukase is a type of imperial decree)
'four nations as one ultimately' is the wordplay.
'four nations' becomes 'UK' (the UK is made up of four nations).
'ultimately' indicates one should take the final letters.
The final letter of 'one' is 'e'.
'uk'+'as'+'e'='UKASE'
'establishing' is the link.
(Other definitions for ukase that I've seen before include "Russian imperial decree" , "Decree (originally from the Tsar)" , "Old Russian edict" , "Tsar's edict" , "Decree or edict from Russia" .)