Blockhead starts to aggravate serious situation (3)
I believe the answer is:
'blockhead' is the definition.
(I've seen this in another clue)
'starts to aggravate serious situation' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'aggravate serious situation' is 'ASS'.
(Other definitions for ass that I've seen before include "Idiot; animal" , "Nincompoop" , "transporter" , "Horse-type animal" , "Turkey" .)