Churchman going round a nuclear plant (7)
I believe the answer is:
reactor
'nuclear plant' is the definition.
(I've seen this in another clue)
'churchman going round a' is the wordplay.
'churchman' becomes 'rector' (I've seen this before).
'going round' means one lot of letters goes inside another (some letters go around others).
'rector' placed around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Producer of nuclear power" , "nuclear vessel" , "I swing back" , "device that's energy-producing" , "one liable to have a meltdown if things go disastrously?" .)