European politician, sovereign or ruler (7)
I believe the answer is:
emperor
'ruler' is the definition.
(emperor is a kind of ruler)
'european politician sovereign or' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'politician' becomes 'MP' (Member of Parliament).
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'e'+'mp'+'er'+'or'='EMPEROR'
(Other definitions for emperor that I've seen before include "Say, Nero" , "Sovereign and richly-colored butterfly" , "Kaiser" , "Supreme ruler" , "Character in 24 ac" .)