Fireplace, something cold or warm, then hot (6)
I believe the answer is:
hearth
'fireplace' is the definition.
(I know that hearth is a type of fireplace)
'then hot' is the wordplay.
'then' becomes 'heart' (I can't justify this - if you can you should believe this answer much more).
'hot' becomes 'h' (abbreviation eg on taps).
'heart'+'h'='HEARTH'
'something cold or warm' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for hearth that I've seen before include "Fireplace surround" , "Floor of a fireplace in the home" , "Novel" , "cosy setting" , "a warm, welcoming place" .)