Youngster somewhat lacking as a pupil (3)

I believe the answer is:

pup

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'youngster' is the definition.
(a pup is a young dog)

I can't explain the rest of the clue.

(Other definitions for pup that I've seen before include "Young dog as palindrome" , "Young animal" , "Canine young" , "Small palindromic dog" , "Young of various animals" .)

I've seen this clue in the Evening Standard.

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