Nuclear device, American, described by minister (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(nuclear reactor)
'american described by minister' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'described by' indicates putting letters inside (to describe can mean to form an outline).
'minister' becomes 'rector' (synonyms).
'a' going within 'rector' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Nuclear power generator" , "I swing back" , "source of power, perhaps" , "device for generating energy" , "Nuclear apparatus for producing energy" .)