I think the best way is ti write the japanese term in romaji, Nibandashi, together with the explication of the word in Italian, Brodo di base di secondo utilizzo. You can simply translate Secondo brodo, it depends on the explication that you give after.
Important: Nibandashi is not made from the reserved liquid of the Ichiban dashi, but it is made using the filtered ingredients (konbu, katsuobushi) used for making the first dashi.