Clergyman touring a nuclear plant (7)
I believe the answer is:
reactor
'nuclear plant' is the definition.
(I've seen this in another clue)
'clergyman touring a' is the wordplay.
'clergyman' becomes 'rector' (rector is a kind of clergyman).
'touring' is an insertion indicator (touring can mean going round).
'rector' enclosing 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "one hosting moderator?" , "Apparatus for production of nuclear energy" , "nuclear facility" , "See 3 Down" , "power supplier" .)