Train carrying post around assembly (10)
I believe the answer is:
parliament
'assembly' is the definition.
(parliament is a kind of assembly)
'train carrying post around' is the wordplay.
'train' becomes 'parent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'carrying' is an insertion indicator.
'post' becomes 'mail' (synonyms).
'around' says the letters should be written backwards.
'mail' in reverse letter order is 'liam'.
'parent' enclosing 'liam' is 'PARLIAMENT'.
Can you help me to learn more?
(Other definitions for parliament that I've seen before include "Legislative assembly" , "Legislative body" , "Diet" , "The law is laid down here" , "Houses" .)