Power of Government's leader in control (5)
I believe the answer is:
reign
'power' is the definition.
(reign can mean time in power)
'government's leader in control' is the wordplay.
'leader' indicates taking the first letters.
'in' is an insertion indicator.
'control' becomes 'rein' (rein is a kind of control).
The first letter of 'governments' is 'g'.
'g' placed inside 'rein' is 'REIGN'.
'of' acts as a link.
(Other definitions for reign that I've seen before include "Period on the throne" , "Monarch's rule" , "Control" , "Period of a monarchy" , "Rule as king or queen" .)