Black inside, rotten old wheat in lethal buffet (9)
I believe the answer is:
deathblow
'lethal buffet' is the definition.
(buffet can mean a hit or blow)
'black inside rotten old wheat' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'inside' means one lot of letters goes inside another.
'rotten' indicates anagramming the letters.
'old'+'wheat'='oldwheat'
'oldwheat' with letters rearranged gives 'deathlow'.
'b' put inside 'deathlow' is 'DEATHBLOW'.
'in' is the link.
(Other definitions for deathblow that I've seen before include "Sudden life-destroying event" , "Coup de grace" .)