Blockhead starts with amazingly silly speech (3)
I believe the answer is:
ass
'blockhead' is the definition.
(I've seen this before)
'starts with amazingly silly speech' is the wordplay.
'starts with' says to take the initial letters.
The first letters of 'amazingly silly speech' is 'ASS'.
(Other definitions for ass that I've seen before include "Dipstick" , "Jennet" , "jack" , "Charlie" , "Small hoofed animal" .)