Youngster in regalia (3)
I believe the answer is:
gal
'youngster' is the definition.
'in regalia' is the wordplay.
'in' says the answer is hidden in the clue.
'GAL' is hidden within 'regalia'.
(Other definitions for gal that I've seen before include "female noted in Kalamazoo" , "An American girl, cowboy vintage" , "Young woman (colloq.)" , "measure of acceleration?" , "Girl from American west" .)