Peg, callous sister upset youngster (8)

I believe the answer is:

teenager

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'youngster' is the definition.
(I know that teenager is a type of youngster)

'peg callous sister upset' is the wordplay.
'peg' becomes 'tee' (tee is a kind of peg).
'callous sister upset' becomes 'nager' (I can't justify this - if you can you should give a lot more credence to this answer).
'tee'+'nager'='TEENAGER'

Can you help me to learn more?

(Other definitions for teenager that I've seen before include "Generate (anag)" , "one that's not yet made a score?" , "Young person" , "hormonal problem?" , "troublesome youth" .)

I've seen this clue in The Times.

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