Position of control gained by the superior worker (5,4)
I believe the answer is:
'position of control' is the definition.
'upper hand' can be an answer for 'control' (I've seen this before). I am unsure of the 'position of' bit.
'the superior worker' is the wordplay.
'the superior' becomes 'upper' (eg an upper house in a parliament).
'worker' becomes 'hand' (hand is a kind of worker).
'gained by' acts as a link.
Can you help me to learn more?
(Other definitions for upper hand that I've seen before include "superiority" , "Position of advantage and control" , "Mastery, advantage" , "Superior position" , "not usually right for driver" .)