Thrash black sheep (4)
I believe the answer is:
lamb
'sheep' is the definition.
(I know that sheep can be written as lamb)
'thrash black' is the wordplay.
'thrash' becomes 'lam' (both can mean to hit).
'black' becomes 'b' (as in pencil classifications).
'lam'+'b'='LAMB'
(Other definitions for lamb that I've seen before include "A follower" , "Young farm animal" , "Noted essayist - symbol of innocence" , "Charles ---, essayist; meek animal" , "Young creature" .)