Cleric holding a nuclear device (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(nuclear reactor)
'cleric holding a' is the wordplay.
'cleric' becomes 'rector' (type of clergyman).
'holding' means one lot of letters goes inside another.
'rector' going around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Nuclear energy source" , "Nuclear apparatus for producing energy" , "Atomic pile" , "nuclear vessel" , "I swing back" .)