Principle initially established in marquee (5)
I believe the answer is:
tenet
'principle' is the definition.
(a tenet is a principle or value)
'initially established in marquee' is the wordplay.
'initially' says to take the initial letters.
'in' indicates putting letters inside.
'marquee' becomes 'tent' (I've seen this in another clue).
The initial letter of 'established' is 'e'.
'e' inserted into 'tent' is 'TENET'.
(Other definitions for tenet that I've seen before include "Belief or doctrine - a palindrome" , "Doctrine; principle held" , "A belief or credo - it's a palindrome" , "Article of faith; doctrine" , "It's believed" .)