Idiotic content of a letter (5)
I believe the answer is:
inane
'idiotic' is the definition.
(I've seen this before)
'content of a letter' is the wordplay.
'content of' becomes 'in' (I can't explain this - if you can you should give a lot more credence to this answer).
'a' becomes 'an'.
'letter' becomes 'e'.
'in'+'an'+'e'='INANE'
Can you help me to learn more?
(Other definitions for inane that I've seen before include "Devoid of intelligence" , "Without meaning or point" , "Thick" , "Idiotic" , "unimaginitive" .)