Einstein an egghead, not entirely silly (5)
I believe the answer is:
inane
'silly' is the definition.
(thesaurus)
'einstein an egghead not entirely' is the wordplay.
'not entirely' indicates a hidden word.
'INANE' is hidden in the letters of 'einstein an egghead'.
(Other definitions for inane that I've seen before include "Daft" , "Without meaning or point" , "Annie (anag.)" , "Moronic" , "Lacking meaning" .)