Coarse-grained rock (7)
I believe the answer is:
granite
'coarse-grained rock' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for granite that I've seen before include "Igneous rock, generally composed of feldspar and mica and quartz" , "Coarse-grained igneous rock" , "Constituent in Dartmoor" , "Hard granular rock" , "A very hard rock from Tangier" .)