Make assistant, keeping in control (8)
I believe the answer is:
dominate
'control' is the definition.
(dominating is a kind of controlling)
'make assistant keeping in' is the wordplay.
'make' becomes 'do' (doing is a kind of making).
'assistant' becomes 'mate' (I've seen this before).
'keeping' indicates putting letters inside.
'mate' enclosing 'in' is 'minate'.
'do'+'minate'='DOMINATE'
(Other definitions for dominate that I've seen before include "Master" , "Govern" , "corner" , "Command, control" , "Admit one in order to be in control" .)