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' means one lot of letters goes inside another.
'post' becomes 'mail' (synonyms).
'around' shows that the letters should be reversed in order.
'mail' back-to-front 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" .)