Freezing, as having lifted top (5)
I believe the answer is:
gelid
'freezing' is the definition.
(synonyms)
'as having lifted top' is the wordplay.
'as' becomes 'eg' (I've seen this in other clues).
'having lifted' shows that the letters should be reversed in order (in a down clue, letters go up).
'top' becomes 'lid' (lid is a kind of top).
'eg' reversed gives 'ge'.
'ge'+'lid'='GELID'
(Other definitions for gelid that I've seen before include "Icy cold - glide (anag)" , "Extremely cold, frozen" , "Glacial" .)