Extreme coldness of nice sis changed for the worse (7)
I believe the answer is:
iciness
'extreme coldness' is the definition.
'nice sis changed for the worse' is the wordplay.
'changed for the worse' indicates an anagram (a worse spelling of the original letters).
'nice'+'sis'='nicesis'
'nicesis' with letters rearranged gives 'ICINESS'.
'of' is the link.
(Other definitions for iciness that I've seen before include "Hostility, unfriendliness" , "Hostile or cold nature" , "Chill; reserve" , "Extreme cold" , "Hostile attitude" .)