Congress competently takes on rising disorder (8)
I believe the answer is:
assembly
'congress' is the definition.
(thesaurus)
'competently takes on rising disorder' is the wordplay.
'competently' becomes 'ably' (synonyms).
'takes on' indicates putting letters inside.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'disorder' becomes 'mess' (messing is a kind of disordering).
'mess' in reverse letter order is 'ssem'.
'ably' going around 'ssem' is 'ASSEMBLY'.
(Other definitions for assembly that I've seen before include "Gathering of people or fitting of parts together" , "Gathering of people for a common purpose" , "Gathering, congregation" , "Religious gathering" , "Legislative gathering" .)