A dry, mostly freezing, upper room (5)
I believe the answer is:
attic
'upper room' is the definition.
(I've seen this before)
'a dry mostly freezing' is the wordplay.
'dry' becomes 'tt' (abbreviation for teetotal).
'mostly' means to remove the last letter.
'freezing' becomes 'icy' ('icy' can be a synonym of 'freezing').
'icy' with its final letter taken away is 'ic'.
'a'+'tt'+'ic'='ATTIC'
(Other definitions for attic that I've seen before include "cockloft" , "Athenian - garret" , "Top chamber" , "Classical" , "Top storey" .)