European in marquee establishes principle (5)
I believe the answer is:
tenet
'principle' is the definition.
(I know that principle can be written as tenet)
'european in marquee' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'in' indicates putting letters inside.
'marquee' becomes 'tent' (I've seen this in another clue).
'e' inserted within 'tent' is 'TENET'.
'establishes' acts as a link.
(Other definitions for tenet that I've seen before include "Article of belief" , "Dogma or principle" , "Teaching" , "Fundamental belief" , "A belief or credo - it's a palindrome" .)