A dry, mostly freezing cold, upper room (5)
I believe the answer is:
attic
'upper room' is the definition.
(I've seen this before)
'a dry mostly freezing cold' is the wordplay.
'dry' becomes 'tt' (abbreviation for teetotal).
'mostly' means to remove the last letter.
'freezing cold' becomes 'icy' (I've seen this before).
'icy' with its final letter taken away is 'ic'.
'a'+'tt'+'ic'='ATTIC'
(Other definitions for attic that I've seen before include "cockloft" , "Loft room" , "Area at the top of the house" , "Tacit (anag.)" , "Room at the top of a house" .)