Idiot ostensibly asinine, foolish primarily (3)
I believe the answer is:
oaf
'idiot' is the definition.
(I've seen this before)
'ostensibly asinine foolish primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'ostensibly asinine foolish' is 'OAF'.
(Other definitions for oaf that I've seen before include "one of thick character" , "Yahoo" , "Goon" , "Boor or lout" , "buffoon" .)