Marx comes across silver in city (7)
I believe the answer is:
chicago
'city' is the definition.
(Chicago is an example)
'marx comes across silver' is the wordplay.
'marx' becomes 'chico' (chico marx was a marx brother).
'comes across' is an insertion indicator.
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'chico' placed around 'ag' is 'CHICAGO'.
'in' is the link.
(Other definitions for chicago that I've seen before include "Port on Lake Michigan" , "Al Capone's city" , "Musical (in Illinois?)" , "A Windy City in Illinois" , "City on Lake Michigan" .)