Model girl, one among hundreds (7)
I believe the answer is:
classic
'model' is the definition.
(I've seen this before)
'girl one among hundreds' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'one among' indicates putting letters inside.
'hundreds' becomes 'cic' (I can't explain this - if you can you should give a lot more credence to this answer).
'lass' going within 'cic' is 'CLASSIC'.
Can you help me to learn more?
(Other definitions for classic that I've seen before include "Relating to early Greek and Latin writers" , "Creation of the highest standard" , "First-class" , "Accepted as excellent" , "The Derby or The Oaks, for example" .)