Hot planet's fireplaces (7)
I believe the answer is:
hearths
'fireplaces' is the definition.
('hearth' can be a synonym of 'fireplace')
'hot planet's' is the wordplay.
'hot' becomes 'h' (abbreviation eg on taps).
'planet's' becomes 'earth's' (Earth is an example).
'h'+'earths'='HEARTHS'
(Other definitions for hearths that I've seen before include "Fireplace areas" , "Firesides" .)