Youngster taking in a model (3,2)
I believe the answer is:
pin up
'a model' is the definition.
(pin-up can mean a model)
'youngster taking in' is the wordplay.
'youngster' becomes 'pup' (a pup is a young dog).
'taking' indicates putting letters inside.
'pup' enclosing 'in' is 'PIN-UP'.
(Other definitions for pin up that I've seen before include "Picture" , "glamour girl" , "Dish" , "Idol" , "Scantily-clad beauty" .)