Cover in outbuilding not quite retaining warmth (7)
I believe the answer is:
sheathe
'cover' is the definition.
(to sheathe something is to cover it)
'outbuilding not quite retaining warmth' is the wordplay.
'outbuilding' becomes 'shed' (shed is a kind of outbuilding).
'not quite' means to remove the last letter (most but not all of the word).
'retaining' is an insertion indicator (inserted letters are held or retained).
'warmth' becomes 'heat' (synonyms).
'shed' with its final letter removed is 'she'.
'she' enclosing 'heat' is 'SHEATHE'.
'in' is the link.
(Other definitions for sheathe that I've seen before include "Put away (a sword)" , "Put (a sword) it its cover" , "Put into scabbard" , "Put (a sword) into case" , "Protective cover, for sword say" .)