Foolish, fatuous (5)
I believe the answer is:
inane
This clue is a double definition.
'foolish' is the first definition.
(inane can mean silly or foolish)
'fatuous' is the second definition.
(I know that fatuous can be written as inane)
(Other definitions for inane that I've seen before include "Silly, witless" , "Idiotic" , "Dull-witted" , "Moronic" , "unimaginative" .)