Pretend not to notice din with tune barely altered (4,1,5,3)
I believe the answer is:
turn a blind eye
'pretend not to notice' is the definition.
(I've seen this before)
'din with tune barely altered' is the wordplay.
'with' means one lot of letters go next to another.
'altered' indicates anagramming the letters.
'tune'+'barely'='tunebarely'
'din' put after 'tunebarely' is 'tunebarelydin'.
'tunebarelydin' is an anagram of 'TURN A BLIND EYE'.
(Other definitions for turn a blind eye that I've seen before include "Affect not to see" , "Affect not to notice" , "take no notice" , "Pretend not to notice" , "Deliberately be oblivious" .)