Clergyman hides a nuclear device (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(I've seen this before)
'clergyman hides a' is the wordplay.
'clergyman' becomes 'rector' (rector is a kind of clergyman).
'hides' is an insertion indicator.
'rector' enclosing 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "chemical substance!" , "one liable to have a meltdown if things go disastrously?" , "Nuclear energy source" , "See 3 Down" , "Producer of nuclear energy" .)