Fireplace lacking top soil (5)
I believe the answer is:
earth
'soil' is the definition.
(synonyms)
'fireplace lacking top' is the wordplay.
'fireplace' becomes 'hearth' (synonyms).
'lacking top' means to remove the first letter.
'hearth' with its initial letter taken off is 'EARTH'.
(Other definitions for earth that I've seen before include "Soil; planet" , "cost of which is very high" , "Soil; fox's lair" , "hideaway" , "brownish pigment?" .)