Builder not initially shielding a nuclear device (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(nuclear reactor)
'builder not initially shielding a' is the wordplay.
'builder' becomes 'erector' (I've seen this before).
'not initially' means to remove the first letter.
'shielding' indicates putting letters inside.
'erector' with its initial letter removed is 'rector'.
'rector' placed around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "device that's energy-producing" , "nuclear vessel" , "Creator (anag.)" , "source of power, perhaps" , "Nuclear power generator" .)