Intelligent European principle (9)
I believe the answer is:
rationale
'principle' is the definition.
('rationale' can be a synonym of 'principle')
'intelligent european' is the wordplay.
'intelligent' becomes 'rational' ('rational' can be similar in meaning to 'intelligent').
'european' becomes 'e' (abbreviation e.g. EU).
'rational'+'e'='RATIONALE'
(Other definitions for rationale that I've seen before include "Underlying principle" , "Logic, reasoning" , "Reasoned exposition" , "This might account for" , "Logical reasons (for action)" .)