Clergyman holding a source of power (7)
I believe the answer is:
reactor
'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 "Device to produce energy by nuclear fission" , "See 3 Down" , "Nuclear power generator" , "Nuclear energy source" , "one hosting moderator?" .)