Express belief about Capone being stony (7)
I believe the answer is:
opaline
'stony' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'express belief about capone' is the wordplay.
'express belief' becomes 'opine' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'capone' becomes 'Al' (gangster Al Capone).
'opine' enclosing 'al' is 'OPALINE'.
'being' acts as a link.
Can you help me to learn more?
(Other definitions for opaline that I've seen before include "Milk glass" , "not entirely clear" , "Translucent glass" , "Photographic plate" , "like a stone" .)