Pin-up by a youngster (6)
I believe the answer is:
nipper
'youngster' is the definition.
('nipper' can be a synonym of 'youngster')
'pin-up by a' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'by' is a charade indicator (letters next to each other).
'a' becomes 'per'.
'pin' reversed gives 'nip'.
'nip'+'per'='NIPPER'
(Other definitions for nipper that I've seen before include "Young person (with claws?)" , "One pinches" , "Small child or crab's claw" , "Child (informal); one biting" , "Child - ankle-biter?" .)