Clergyman holding a source of power (7)
I believe the answer is:
Here is my best explanation:
'source of power' is the definition.
'clergyman holding a' is the wordplay.
'clergyman' becomes 'rector' (rector is a kind of clergyman).
'holding' indicates putting letters inside.
'rector' going around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "nuclear vessel", "Creator (anag.)", "(Nuclear) powerplant", "Heart of a nuclear power station", "Nuclear energy producer", "Provider of nuclear power", "Device to produce energy by nuclear fission", "Nuclear energy generator", "Nuclear apparatus for producing energy" and "See 3 Down".)