Philosophy briefly for every single European leader (9)
I believe the answer is:
taoiseach
'european leader' is the definition.
(the Taoiseach is the Irish head of government)
'philosophy briefly for every single' is the wordplay.
'philosophy' becomes 'Taoism' (Taoism is an example).
'briefly' means to remove the last letter.
'for every single' becomes 'each' (for each person).
'taoism' with its final letter taken away is 'taois'.
'taois'+'each'='TAOISEACH'
(Other definitions for taoiseach that I've seen before include "Irish PM" , "Western PM" , "PM in Dublin" , "Leader of the government" , "Irish premier" .)