Pupil's cover (3)
I believe the answer is:
lid
'pupil's cover' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lid that I've seen before include "Receptacle cover" , "Cover for box or pot" , "Cover for box or eye" , "Cover from gliders" , "Kind of cover, may be hinged" .)