Blockhead decapitates the bread (3)
I believe the answer is:
oaf
'blockhead' is the definition.
(I've seen this in another clue)
'decapitates the bread' is the wordplay.
'decapitates' means to remove the first letter (I've seen 'decapitated' mean this).
'the bread' becomes 'loaf' (loaf of bread).
'loaf' with its first letter removed is 'OAF'.
(Other definitions for oaf that I've seen before include "Barbarian" , "Lout; dolt" , "A clumsy or stupid person" , "Stupid or loutish person" , "Boorish person" .)