A schoolkid's problem covered by Isaac Newton (4)

I believe the answer is:

acne

'a schoolkid's problem' is the definition.

'covered by isaac newton' is the wordplay.
'covered by' indicates the answer is hidden within the clue.
'ACNE' can be found hidden inside 'isaac newton'.

(Other definitions for acne that I've seen before include "sort of disease" , "Pimply ailment of adolescnce" , "Skin condition prevalent among teenagers" , "Problem to face in" , "Cane for the teenager?" .)

I've seen this clue in The Times.

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