Discontented smith retains warmth using protective covering (6)
I believe the answer is:
sheath
'protective covering' is the definition.
(sheath is a kind of protective covering)
'discontented smith retains warmth' is the wordplay.
'discontented' says to hollow out the word (remove centre letters) (the word's contents are removed).
'retains' is an insertion indicator.
'warmth' becomes 'heat' (synonyms).
'smith' with its centre taken out is 'sh'.
'sh' enclosing 'heat' is 'SHEATH'.
'using' acts as a link.
(Other definitions for sheath that I've seen before include "Protective cover" , "Protective case for a knife or sword" , "Holder for a dagger" , "Close fitting dress" , "coat" .)