Guard for pupil that's batted sometimes (6)
I believe the answer is:
eyelid
'guard' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I cannot understand how the rest of the clue works.
(Other definitions for eyelid that I've seen before include "Cover for viewer" , "Fold of skin on the face" , "Blinking membrane" , "Covering for organ of sight" , "It may be batted" .)