Cubic crystalline mineral? It is in sound condition! (6)
I believe the answer is:
halite
'cubic crystalline mineral?' is the definition.
'halite' can be an answer for 'mineral?' (halite is a kind of mineral). I'm not certain of the 'cubic crystalline' bit.
'it is in sound condition' is the wordplay.
'is in' means one lot of letters goes inside another.
'sound condition' becomes 'hale' (I am not sure about this - if you are sure you should believe this answer much more).
'it' going inside 'hale' is 'HALITE'.
Can you help me to learn more?
(Another definition for halite that I've seen is " Rock salt".)