That is to slot the name into, silly! (5)
I believe the answer is:
inane
'silly' is the definition.
(inane means silly or foolish)
'that is to slot the name' is the wordplay.
I cannot quite see how this works, but
'name' could be 'nan' and 'nan' is found within the answer.
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is found in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'into' is the link.
Can you help me to learn more?
(Other definitions for inane that I've seen before include "Silly or senseless" , "Devoid of intelligence" , "Witless and pointless" , "Fatuous, asinine" , "Empty-headed" .)