Stone from Ulster incorporated into fireplace (7)
I believe the answer is:
granite
'stone' is the definition.
(granite is a kind of stone)
'ulster incorporated into fireplace' is the wordplay.
'ulster' becomes 'ni' (Northern Ireland).
'incorporated into' is an insertion indicator.
'fireplace' becomes 'grate' (I've seen this before).
'ni' going within 'grate' is 'GRANITE'.
'from' acts as a link.
(Other definitions for granite that I've seen before include "Resilience; stone" , "Coarse-grained igneous rock" , "Tearing (anag.)" , "Igneous rock, generally composed of feldspar and mica and quartz" , "Hard granular rock" .)