The cleric takes in a nuclear device (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(nuclear reactor)
'the cleric takes in a' is the wordplay.
'the cleric' becomes 'rector' (type of clergyman).
'takes in' indicates putting letters inside.
'rector' placed around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Nuclear energy generator" , "Creator (anag.)" , "device for generating energy" , "(Nuclear) powerplant" , "Producer of nuclear energy" .)