Old man perhaps accepting post up in assembly (10)
I believe the answer is:
parliament
'assembly' is the definition.
(I know that assembly can be written as parliament)
'old man perhaps accepting post up' is the wordplay.
'old man perhaps' becomes 'parent' (old man can mean someone's father).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'post' becomes 'mail' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'mail' in reverse letter order is 'liam'.
'parent' enclosing 'liam' is 'PARLIAMENT'.
'in' is the link.
(Other definitions for parliament that I've seen before include "UK legislature" , "Diet" , "Legislative body" , "Legislative assembly" , "Houses" .)