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Barrier
dropped
to
protect
pupil
(6)
Barrier dropped to protect pupil (6)
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The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue is a cryptic definition. An eyelid is indeed a barrier that drops to protect the pupil — but the misdirection here is that you think of a school pupil, when in fact it's the pupil of the eye.