May must accept European sovereignty, being blue (8)
I believe the answer is:
cerulean
'blue' is the definition.
(I know that cerulean is a type of blue)
'may must accept european sovereignty being' is the wordplay.
'may' becomes 'can' (synonyms).
'must accept' means one lot of letters goes inside another.
'european' becomes 'E' (abbreviation e.g. EU).
'sovereignty being' becomes 'rule' (I am not sure about this - if you are sure you should believe this answer much more).
'e'+'rule'='erule'
'can' enclosing 'erule' is 'CERULEAN'.
Can you help me to learn more?
(Other definitions for cerulean that I've seen before include "Azure" , "Deep blue colour" , "Light shade of blue" , "Sky-blue for Laurence" , "of clear heavenly appearance" .)