Dullard’s drill (4)
I believe the answer is:
bore
This clue is a double definition.
'dullard's' is the first definition.
(synonyms)
'drill' is the second definition.
(I know that drill can be written as bore)
(Other definitions for bore that I've seen before include "Drill - pest" , "Hollow out; dull person" , "No fun" , "Diameter of a gun barrel" , "Tedious chap" .)