Lassaigne's extract of p-nitrochlorobenzene is acidified with $\mathrm{dil}.{\mathrm{HNO}}_3$ and then treated with silver nitrate solution, the white precipitate formed is due to

Lassaigne's extract for the compound p-nitrochlorobenzene contains both NaCN as well as NaCl, so on treatment with ${\mathrm{AgNO}}_3,$ the extract gives precipitate of AgCN as well as AgCl unless it is boiled with ${\mathrm{HNO}}_3$ which will remove NaCN as gaseous HCN.