Idiot about a great deal (4)
I believe the answer is:
clot
'idiot' is the definition.
(both can mean a stupid person)
'about a great deal' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'a great deal' becomes 'lot' (both can mean a large amount).
'c'+'lot'='CLOT'
(Other definitions for clot that I've seen before include "Mass of material" , "3 [DUMMY]" , "Thick lump - thick person" , "(Of blood) solidify" , "Silly billy" .)