Rock and nickel in fireplace (7)
I believe the answer is:
granite
'rock' is the definition.
(granite is a kind of rock)
'nickel in fireplace' is the wordplay.
'nickel' becomes 'ni' (Ni is the chemical symbol for nickel).
'in' is an insertion indicator.
'fireplace' becomes 'grate' (I've seen this before).
'ni' going inside 'grate' is 'GRANITE'.
'and' is the link.
(Other definitions for granite that I've seen before include "Resilience; stone" , "Coarse-grained igneous rock" , "Hard igneous rock" , "Crystalline rock" , "Hard granular rock" .)