Schoolchild is essential for vision (5)

I believe the answer is:

pupil

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'schoolchild' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

(Other definitions for pupil that I've seen before include "one may see through!" , "that shrinks from brightness" , "One in the eye or in place of learning" , "One studying" , "Trainee barrister" .)

I've seen this clue in the Evening Standard.

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