Cuddling grandma, that is silly (5)
I believe the answer is:
inane
'silly' is the definition.
(I've seen this before)
'cuddling grandma that is' is the wordplay.
'cuddling' indicates putting letters inside (inserted letters are embraced or welcomed in).
'grandma' becomes 'nan' (I've seen this in other clues).
'that is' becomes 'ie' (synonyms).
'nan' inserted within 'ie' is 'INANE'.
(Other definitions for inane that I've seen before include "Empty-headed" , "Stupid and silly" , "Fatuous, mindless" , "Really stupid" , "Frivolous" .)