Bitterly cold for one retracting cover (5)
I believe the answer is:
gelid
'bitterly cold' is the definition.
(thesaurus)
'for one retracting cover' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'retracting' shows that the letters should be reversed in order (I've seen 'retracted' mean this).
'cover' becomes 'lid' (lid is a kind of cover).
'eg' back-to-front is 'ge'.
'ge'+'lid'='GELID'
(Other definitions for gelid that I've seen before include "Extremely cold, frozen" , "Glacial" , "Icy cold - glide (anag)" .)