For the snow, an efficient but expensive warmth retainer (5)
I believe the answer is:
sable
'snow an efficient but expensive warmth retainer' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sable that I've seen before include "Bales of expensive Russian fur" , "Bales of valuable Russian fur" , "Bales of rich fur" , "Black (heraldry); fur" , "Antelope, fur" .)