Rock inverted in filling fireplace (7)
I believe the answer is:
granite
'rock' is the definition.
(type of rock)
'inverted in filling fireplace' is the wordplay.
'inverted' is a reversal indicator.
'filling' means one lot of letters goes inside another.
'fireplace' becomes 'grate' (grate can mean a fireplace).
'in' backwards is 'ni'.
'ni' placed into 'grate' is 'GRANITE'.
(Other definitions for granite that I've seen before include "Showing great resolution" , "determination" , "Very hard type of rock" , "Resilience; stone" , "Crystalline rock" .)