Minister covering area is a source of power (7)
I believe the answer is:
reactor
'a source of power' is the definition.
'minister covering area' is the wordplay.
'minister' becomes 'rector' ('rector' can be a synonym of 'minister').
'covering' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
'rector' going around 'a' is 'REACTOR'.
'is' acts as a link.
(Other definitions for reactor that I've seen before include "Nuclear apparatus for producing energy" , "source of power, perhaps" , "nuclear facility" , "Creator (anag.)" , "(Nuclear) powerplant" .)