City on the Po where Guarneri, Stradivari and Amati made instruments (7)
I believe the answer is:
cremona
(Other definitions for cremona that I've seen before include "where Stradivari made violins" , "Guarneri's place" , "here in Lombardy" , "in Italy" , "...and the Italian home of both Stradivari and the Amatis" .)