Extremely wicked act at Beijing, say, involving soldiers after revolution (8)
I believe the answer is:
atrocity
'extremely wicked act' is the definition.
(I've seen this before)
'at beijing say involving soldiers after revolution' is the wordplay.
'beijing say' becomes 'city' (beijing is a kind of city).
'involving' is an insertion indicator.
'soldiers' becomes 'or' (Other Rank).
'after revolution' shows that the letters should be reversed in order.
'or' reversed gives 'ro'.
'at'+'city'='atcity'
'atcity' enclosing 'ro' is 'ATROCITY'.
(Other definitions for atrocity that I've seen before include "Wickedness" , "Frightful act of citatory kind" , "Outrage" , "Heinous deed" , "Appalling act" .)