### Clue: Cold, small river submerged two fish-heads (7)
### Answer: SNIFFLE 
 ### Explanation: Definition: Cold
 S(abbrev. for "small") + NILE(the African river) **containing**(submerged) **two of the 1st letter of**(…-heads) "fish ".
Answer:  I think the plural form is used when referring to a cold.