Clergyman welcoming a nuclear facility (7)
I believe the answer is:
reactor
'nuclear facility' is the definition.
(I've seen this before)
'clergyman welcoming a' is the wordplay.
'clergyman' becomes 'rector' (rector is a kind of clergyman).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'rector' placed around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Device to produce energy by nuclear fission" , "Producer of nuclear power" , "See 3 Down" , "nuclear vessel" , "nuclear facility" .)