Decree this country’s European borders American (5)
I believe the answer is:
ukase
'decree' is the definition.
(ukase is a kind of decree)
'country's european borders american' is the wordplay.
'country's' becomes 'UK's' (United Kingdom).
'european' becomes 'E' (abbreviation e.g. EU).
'borders' is an insertion indicator (inserted letters are bordered by others).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'uks'+'e'='ukse'
'ukse' placed around 'a' is 'UKASE'.
'this' acts as a link.
(Other definitions for ukase that I've seen before include "Old Russian edict" , "Formerly, edict of the Russian tsar" , "Official or arbitrary decree" , "Official decree, from Russia" , "Russian imperial decree" .)