Like simpleton's head, idiot!
I believe the answer is:
ass
'idiot' is the definition.
(I've seen this before)
'like simpleton's head' is the wordplay.
'like' becomes 'as'.
'head' suggests taking the first letters.
The first letter of 'simpletons' is 's'.
'as'+'s'='ASS'
(Other definitions for ass that I've seen before include "sap" , "jack" , "twit!" , "Stupid fellow" , "Silly fool or beast of burden" .)