Carter wrong to carry round nuclear device (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(nuclear reactor)
'carter wrong to carry round' is the wordplay.
'wrong' indicates anagramming the letters.
'to carry' indicates putting letters inside.
'round' becomes 'o' (o is a circular letter).
'carter' with letters rearranged gives 'reactr'.
'reactr' enclosing 'o' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "nuclear vessel" , "Nuclear power source" , "Creator (anag.)" , "one liable to have a meltdown if things go disastrously?" , "(Nuclear) powerplant" .)