Ridiculous brain an egghead possesses (5)
I believe the answer is:
inane
'ridiculous' is the definition.
(thesaurus)
'brain an egghead possesses' is the wordplay.
'possesses' indicates a hidden word.
'INANE' is hidden in the letters of 'brain an egghead'.
(Other definitions for inane that I've seen before include "Frivolous" , "Silly, witless" , "Lacking meaning" , "Fatuous, asinine" , "Fatuous, vacuous" .)