### Clue: O Kelvin, it's cold! (8,4)
### Answer: ABSOLUTE ZERO 
 ### Explanation: Definition: it's cold
 Cryptic defn: Reference to 0 degrees on the Kelvin temperature scale (named after scientist William Thomson, aka Lord Kelvin), equal to -273.15 degrees Centigrade, which temperature has been achieved to within a billionth of a degree, and maybe closer still, but never absolutely, in future.