Cold but awfully nice in one vessel (7)
I believe the answer is:
iciness
'cold' is the definition.
(I know that iciness is a type of cold)
'awfully nice in one vessel' is the wordplay.
'awfully' indicates an anagram.
'in' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'nice' is an anagram of 'cine'.
'i'+'ss'='iss'
'cine' going into 'iss' is 'ICINESS'.
'but' is the link.
(Other definitions for iciness that I've seen before include "Frozen state of" , "unfriendly manner" , "Hostile attitude" , "Extreme chill" , "Frostiness" .)