Pool table rock (5)
I believe the answer is:
slate
'pool table rock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slate that I've seen before include "Criticise harshly" , "Scold; hard rock" , "A roof-tile; criticise heavily" , "Small blackboard - for deferred payments" , "Bluish-grey colour" .)