Family man holds up letters for national assembly (10)
I believe the answer is:
parliament
'for national assembly' is the definition.
(I know that National Assembly is an parliament)
'family man holds up letters' is the wordplay.
'family man' becomes 'parent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'holds' means one lot of letters goes inside another.
'up' is a reversal indicator (in down clue: letters go upwards).
'letters' becomes 'mail' (I've seen this before).
'mail' reversed gives 'liam'.
'parent' enclosing 'liam' is 'PARLIAMENT'.
Can you help me to learn more?
(Other definitions for parliament that I've seen before include "Diet" , "UK legislature" , "Houses" , "The law is laid down here" , "Legislative body" .)