Not stupid about silver being immensely popular (3,3,4)
I believe the answer is:
all the rage
'immensely popular' is the definition.
'not stupid about silver' is the wordplay.
'not stupid' becomes 'allthere' ('not all there' can mean stupid).
'about' indicates putting letters inside.
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'allthere' enclosing 'ag' is 'ALL THE RAGE'.
'being' acts as a link.
(Other definitions for all the rage that I've seen before include "Fashionable" , "Very much in fashion" , "most desirable now!" , "ultra-fashionable" , "Extremely popular" .)