Very cold metal in jumbo (6)
I believe the answer is:
biting
'very cold' is the definition.
'metal in jumbo' is the wordplay.
'metal' becomes 'tin' (tin is a kind of metal).
'in' means one lot of letters goes inside another.
'jumbo' becomes 'big' (similar in meaning).
'tin' inserted into 'big' is 'BITING'.
(Other definitions for biting that I've seen before include "Mordant" , "Painful or stinging" , "(Of wit) harsh, cruel" , "Caustic or cutting - like teeth?" , "Cold - tart" .)