Important to keep can very cold (6)
I believe the answer is:
biting
'very cold' is the definition.
'important to keep can' is the wordplay.
'important' becomes 'big' (synonyms).
'to keep' is an insertion indicator.
'can' becomes 'tin' (both are metal containers).
'big' going around 'tin' is 'BITING'.
(Other definitions for biting that I've seen before include "Cold - tart" , "Caustic or cutting - like teeth?" , "Painful or stinging" , "Keen" , "Acerbic" .)