Youngster doesn't finish party (3)

I believe the answer is:

gal

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'youngster' is the definition.

'doesn't finish party' is the wordplay.
'doesn't finish' becomes 'ga' (I am not sure about this - if you are sure you should believe this answer much more).
'party' becomes 'l'.
'ga'+'l'='GAL'

Can you help me to learn more?

(Other definitions for gal that I've seen before include "US female" , "measure of acceleration?" , "An American girl, cowboy vintage" , "Girl from American west" , "miss" .)

I've seen this clue in The Mirror.

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