Refuse to hug model for body (7)
I believe the answer is:
density
'for body' is the definition.
(density is a kind of body)
'refuse to hug model' is the wordplay.
'refuse' becomes 'deny' (synonyms).
'to hug' means one lot of letters goes inside another.
'model' becomes 'sit' (e.g. for a painting).
'deny' going around 'sit' is 'DENSITY'.
(Other definitions for density that I've seen before include "Closeness" , "Concentration, compactness" , "can be relative" , "showing obtuseness" , "The compactness of a substance" .)