Controller is upset about mercenary seizing Government (8)
I believe the answer is:
svengali
'controller' is the definition.
(term for a controlling figure)
'upset about mercenary seizing government' is the wordplay.
'upset' becomes 'si' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'mercenary' becomes 'venal' (both can mean purely concerned with making money).
'seizing' is an insertion indicator.
'government' becomes 'g' (abbreviation for government).
'venal' enclosing 'g' is 'vengal'.
'si' placed around 'vengal' is 'SVENGALI'.
'is' acts as a link.
(Other definitions for svengali that I've seen before include "in novel form" , "Person who tries to force another to do his bidding" , "Evil manager" , "George du Maurier character who controls Trilby" , "evil manipulator" .)