Not crazy to collect silver — it's very popular (3,3,4)
I believe the answer is:
all the rage
'very popular' is the definition.
(I've seen this before)
'not crazy to collect silver' is the wordplay.
'not crazy' becomes 'all there' (as opposed to 'not all there').
'to collect' is an insertion indicator (inserted letters are collected by others).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'allthere' enclosing 'ag' is 'ALL THE RAGE'.
'it's' acts as a link.
(Other definitions for all the rage that I've seen before include "The in thing" , "Very popular (informal)" , "Extremely popular" , "in vogue?" , "ultra-fashionable" .)