Blockhead with extra-busy diary to announce? (5)
I believe the answer is:
moron
'blockhead' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for moron that I've seen before include "Person of subnormal intelligence" , "Simpleton" , "Idiot" , "Clot" , "Wally" .)