Cold, as night unfortunately closes in -- bitter (8)
I believe the answer is:
scathing
'bitter' is the definition.
(synonyms)
'cold as night unfortunately closes' is the wordplay.
'cold' becomes 'c' (eg on taps).
'unfortunately' indicates an anagram.
'closes' is an insertion indicator (inserted letters are closed or sealed in).
'as'+'night'='asnight'
'asnight' is an anagram of 'sathing'.
'c' put within 'sathing' is 'SCATHING'.
'in' is the link.
(Other definitions for scathing that I've seen before include "Witheringly scornful" , "Painful" , "Acid" , "Scornfully critical" , "Caustic" .)