Child's organiser contains letters about assembly (10)
I believe the answer is:
parliament
'assembly' is the definition.
(parliament is a kind of assembly)
'child's organiser contains letters about' is the wordplay.
'child's organiser' becomes 'parent' (I can't justify this - if you can you should give a lot more credence to this answer).
'contains' is an insertion indicator.
'letters' becomes 'mail' (letters sent in the post or mail).
'about' says the letters should be written in reverse.
'mail' reversed gives 'liam'.
'parent' placed around 'liam' is 'PARLIAMENT'.
Can you help me to learn more?
(Other definitions for parliament that I've seen before include "UK legislature" , "Diet" , "Legislative body" , "Legislative assembly" , "Houses" .)