Reserve nothing extremely chilly (3-4)

I believe the answer is:

sub-zero

'extremely chilly' is the definition.

'reserve nothing' is the wordplay.
'reserve' becomes 'sub' (I've seen this in other clues).
'nothing' becomes 'zero' ('zero' can be a synonym of 'nothing').
'sub'+'zero'='SUB-ZERO'

(Another definition for sub-zero that I've seen is " Colder than freezing".)

I've seen this clue in The Guardian.

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