Upper hand, control (7)
I believe the answer is:
mastery
This clue is a double definition.
'upper hand' is the first definition.
(thesaurus)
'control' is the second definition.
(I know that control can be written as mastery)
(Other definitions for mastery that I've seen before include "Expert skill" , "dominion" , "superior knowledge" , "Supremacy" , "Grasp" .)