Girl with not so much on the outside — requirement for many a model? (8)
I believe the answer is:
leanness
'requirement for many a model?' is the definition.
(many models are required to be lean or skinny)
'girl with not so much on the outside' is the wordplay.
'girl with' becomes 'anne' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'not so much' becomes 'less' (I've seen this before).
'on the outside' means one lot of letters goes inside another.
'anne' put inside 'less' is 'LEANNESS'.
(Other definitions for leanness that I've seen before include "Thinness" , "Lack of fat" , "thin" .)