Slate black after all (5)
I believe the answer is:
blast
'slate' is the definition.
(I've seen this before)
'black after all' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil hardness).
'after all' becomes 'last' (I've seen this in another clue).
'b'+'last'='BLAST'
(Other definitions for blast that I've seen before include "Severe reprimand; strong gust" , "Effect of explosion" , "Loud noise from a horn" , "Blow up or swear" , "Severely criticise" .)