Initially any short-witted simpleton (3)
I believe the answer is:
ass
'simpleton' is the definition.
(ass is a kind of simpleton)
'initially any short-witted' is the wordplay.
'initially' says to take the initial letters.
'short-witted' becomes 'ss' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'any' is 'a'.
'a'+'ss'='ASS'
Can you help me to learn more?
(Other definitions for ass that I've seen before include "Wally" , "transporter" , "Titania loved one" , "Small hoofed animal" , "Moke" .)