Model youngster eating in (3-2)
I believe the answer is:
pin-up
'model' is the definition.
(pin-up can mean a model)
'youngster eating in' is the wordplay.
'youngster' becomes 'pup' (a pup is a young dog).
'eating' means one lot of letters goes inside another.
'pup' placed around 'in' is 'PIN-UP'.
(Other definitions for pin-up that I've seen before include "Attach (to a notice board)" , "(Picture of) sexually attractive person" , "Attractive celebrity" , "Poster of an attractive person" , "Photographer's model" .)