Pretend not to notice (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)
This is all the clue.
(Other definitions for turn a blind eye that I've seen before include "Pretend not to see" , "Overlook deliberately" , "Ignore -- brush aside" , "Deliberately be oblivious" , "Follow Nelson's example" .)