Very old literary festival turned away louts (6)
I believe the answer is:
yahoos
'louts' is the definition.
(I've seen this in another clue)
'very old literary festival turned away' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'old' becomes 'o' (abbreviation).
'literary festival' becomes 'Hay' (annual literary festival in Wales).
'turned away' says the letters should be written backwards.
'so'+'o'+'hay'='soohay'
'soohay' backwards is 'YAHOOS'.
(Other definitions for yahoos that I've seen before include "Swift's brutish men" , "Woefully entertaining attempts at the Oval" , "Brute people invented by Dean Swift" , "Dean Swift's brutish people" , "Brutish people created by Dean Swift" .)