Something extra put on somewhat icily in terrible cold (7)
I believe the answer is:
codicil
'something extra put on somewhat' is the definition.
I can't judge whether this definition defines the answer.
'icily in terrible cold' is the wordplay.
'icily' becomes 'ici' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'terrible' indicates an anagram.
'cold' is an anagram of 'codl'.
'ici' inserted within 'codl' is 'CODICIL'.
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Modifier to will" , "Addendum to will" , "Additional item appended to a will" , "Addition to a will" , "Appendix to a will" .)