Youngster swallows large part of tooth (4)
I believe the answer is:
pulp
'part of tooth' is the definition.
('pulp' is part of 'tooth')
'youngster swallows large' is the wordplay.
'youngster' becomes 'pup' (I've seen this in other clues**).
'swallows' means one lot of letters goes inside another.
'large' becomes 'l'.
'pup' going around 'l' is 'PULP'.
(Other definitions for pulp that I've seen before include "recycle paper" , "Flesh of fruit - type of fiction" , "Soft fruit flesh" , "Tarantino film with John Travolta" , "Soft fleshy part of a fruit" .)