Blockhead initially dismissing this writer’s humour (6)
I believe the answer is:
dimwit
'blockhead' is the definition.
(I know that blockhead can be written as dimwit)
'initially dismissing this writer's humour' is the wordplay.
'initially' says to take the initial letters.
'this writer's' becomes 'I'm' (the person writing the crossword clues).
'humour' becomes 'wit' (synonyms).
The initial letter of 'dismissing' is 'd'.
'd'+'im'+'wit'='DIMWIT'
(Other definitions for dimwit that I've seen before include "Nit" , "Fool" , "idiot!" , "Silly person (not nitwit)" , "Stupid person" .)