Chalkboard (5)
I believe the answer is:
'chalkboard' 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 "A roof-tile; criticise heavily" , "Run down" , "Roofing material or credit recorder" , "Criticise harshly" , "Rubbish" .)