As cold as cold can be (8,4)
I believe the answer is:
absolute zero
'cold can be' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'as cold' is the wordplay.
'as' becomes 'absolute' (I can't explain this - if you can you should give a lot more credence to this answer).
'cold' becomes 'zero' (I've seen this before).
'absolute'+'zero'='ABSOLUTE ZERO'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for absolute zero that I've seen before include "Minus 273 1/4 C approx." , "Coldest temperature" , "OK?" , "Lowest theoretical temperature" , "can't get lower than this" .)