Hole in Swiss cheese (3)
I believe the answer is:
eye
'hole in swiss cheese' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eye that I've seen before include "1 across, 4 across, for example" , "Orb" , "Look at with interest" , "Oculus" , "where the pupil is seated" .)