Sinister controller (8)
I believe the answer is:
svengali
'sinister controller' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for svengali that I've seen before include "Manipulative person" , "Person exerting sinister control over another" , "One exerting evil mental control over another" , "Trilby's mesmerist" , "Evil hypnotist in the George du Maurier novel, Trilby" .)