Study its strange, unknown solidity (7)
I believe the answer is:
density
'solidity' is the definition.
(density is a kind of solidity)
'study its strange unknown' is the wordplay.
'study' becomes 'den' (I've seen this before).
'strange' indicates anagramming the letters.
'unknown' becomes 'y' (algebra).
'its' anagrammed gives 'sit'.
'den'+'sit'+'y'='DENSITY'
(Other definitions for density that I've seen before include "Number of items in a given area" , "'Thickness, concentration (7)'" , "Bulk" , "Concentration, compactness" , "The quantity of a given area" .)