Idiot of astonishing foolishness, primarily (3)
I believe the answer is:
oaf
'idiot' is the definition.
(I've seen this before)
'of astonishing foolishness primarily' is the wordplay.
'primarily' indicates taking the first letters.
The initial letters of 'of astonishing foolishness' is 'OAF'.
(Other definitions for oaf that I've seen before include "Barbarian" , "one of thick character" , "Boorish person" , "Lout, lummox" , "Awkward stupid person" .)