Boffin (8)
'boffin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for egghead that I've seen before include "Expert" , "Intellectual, studious academic" , "may take a bashing at the breakfast table" , "Very clever person" , "Brainy type" .)
(Other definitions for brainbox that I've seen before include "Skull - clever clogs" , "Einstein?" , "cranium" , "quiz champion?" , "Clever type" .)