Silly, that is, to invite granny inside (5)
I believe the answer is:
inane
'silly' is the definition.
(inane means silly or foolish)
'that is to invite granny inside' is the wordplay.
I cannot quite see how this works, but
'granny' could be 'nan' and 'nan' is present in the answer.
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for inane that I've seen before include "Silly, fatuous" , "Really stupid" , "Unintelligent" , "Witless and pointless" , "Foolish, fatuous" .)