A slander upset assembly (4)
I believe the answer is:
duma
'assembly' is the definition.
(I know that Duma is a type of general assembly)
'a slander upset' is the wordplay.
'slander' becomes 'mud' (as in to drag someone's name through the mud).
'upset' shows that the letters should be reversed in order.
'a'+'mud'='amud'
'amud' backwards is 'DUMA'.
(Other definitions for duma that I've seen before include "council" , "Russian elected parliament" , "Russian legislative assembly" , "Russian state assembly" .)