Foolish grandmother that is put outside (5)
I believe the answer is:
inane
'foolish' is the definition.
(inane can mean silly or foolish)
'grandmother that is put outside' is the wordplay.
'grandmother' becomes 'nan' (nan can informally mean a grandmother).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'put outside' indicates putting letters inside.
'nan' placed within 'ie' is 'INANE'.
(Other definitions for inane that I've seen before include "Silly or senseless" , "Puerile" , "unimaginitive" , "Lacking meaning" , "Annie (anag.)" .)