Pretend not to notice what Nelson would do spinning round? (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)
'what nelson would do spinning round?' is the wordplay.
I cannot really see how this works, but
'would' could be 'd' (I've seen this in other clues) and 'd' is found within the answer.
'round?' could be 'turn' ('turn' can be a synonym of 'round') and 'turn' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for turn a blind eye that I've seen before include "Overlook deliberately" , "Pretend not to have seen" , "Affect not to see" , "Pretend not to see" , "Affect not to notice" .)