Minister securing a source of power (7)
I believe the answer is:
reactor
'source of power' is the definition.
'minister securing a' is the wordplay.
'minister' becomes 'rector' ('rector' can be a synonym of 'minister').
'securing' is an insertion indicator.
'rector' placed around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Producer of nuclear energy" , "Nuclear energy source" , "device that's energy-producing" , "I swing back" , "nuclear vessel" .)