### Clue: Chlorine in Italian river and air around island, in whatever proportion (6,3)
### Answer: POISON GAS 
 ### Explanation: Definition: Chlorine
 [ PO(major Italian river) + SONG(an air;a musical piece for voice) ] **containing**(around) I(abbrev. for "island") + AS(in whatever proportion, as used in similes – I'm not sure of this, but I  think that's an apt example of "as" = "in whatever proportion").
Answer: Description of chlorine, an early chemical weapon.