Degree of compactness (7)
I believe the answer is:
density
'degree of compactness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for density that I've seen before include "Number of items in a given area" , "Compactness - stupidity" , "showing obtuseness" , "Mass/unit vol. (phys.)" , "Closeness" .)