European Head of State hosted by e.g. Boris Johnson (8)
I believe the answer is:
estonian
'european' is the definition.
(person from Estonia)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for estonian that I've seen before include "Native of a Baltic republic" , "Eastern Europe" , "Eastern European" , "Baltic state native" , "European language" .)