Non-standard agreement by UEFA, tho’ consistently overlooked (4)
I believe the answer is:
yeah
'non-standard agreement' is the definition.
('yes', informally)
'by uefa tho' consistently overlooked' is the wordplay.
'consistently overlooked' indicates alternate letters (alternate letters are removed).
'by'+'uefa'+'tho'='byuefatho'
The alternating letters of 'byuefatho' are 'YEAH'.
(Other definitions for yeah that I've seen before include "Correct" , "Absolutely!" , "quite casually" , "'She loves you, ..., ..., ...' (song)" , "The old expression of surprise" .)