Very fashionable and completely sane to collect silver (3,3,4)
I believe the answer is:
all the rage
'very fashionable' is the definition.
(something all the rage is very fashionable)
'completely sane to collect silver' is the wordplay.
'completely sane' becomes 'all there' ('not all there' can mean insane).
'to collect' indicates putting letters inside.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'allthere' enclosing 'ag' is 'ALL THE RAGE'.
'and' acts as a link.
(Other definitions for all the rage that I've seen before include "Very much in fashion" , "The in thing" , "Fashionable" , "Extremely popular" , "Very popular (informal)" .)