Fireplace lined with Ulster stone (7)
I believe the answer is:
granite
'stone' is the definition.
(granite is a kind of stone)
'fireplace lined with ulster' is the wordplay.
'fireplace' becomes 'grate' (I've seen this before).
'lined with' is an insertion indicator (as in being filled or lined with something).
'ulster' becomes 'ni' (Northern Ireland).
'grate' placed around 'ni' is 'GRANITE'.
(Other definitions for granite that I've seen before include "Hard stone from Tangier" , "Hard igneous rock" , "Resilience; stone" , "determination" , "Tearing (anag.)" .)