Rest packed into refuse -- how solid? (7)
I believe the answer is:
density
'rest packed' is the definition.
I can't tell whether this definition defines the answer.
'into refuse how solid?' is the wordplay.
'into' indicates putting letters inside.
'refuse' becomes 'deny' ('deny' can be a synonym of 'refuse').
'how solid?' becomes 'sit' (I can't justify this - if you can you should believe this answer much more).
'deny' placed around 'sit' is 'DENSITY'.
Can you help me to learn more?
(Other definitions for density that I've seen before include "The quantity of a given area" , "can be relative" , "Mass/unit vol. (phys.)" , "Closeness" , "Concentrated condition as of fog" .)