Foolish one (6)
(Other definitions for doofus that I've seen before include "Nitwit" , "American fool - US food (anag)" .)
'foolish one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for twit that I've seen before include "Slangily a foolish person" , "Silly fool" , "Bait - fool" , "Idiot; tease" , "Blockhead" .)