Cleric astride American nuclear device (7)
I believe the answer is:
reactor
'nuclear device' is the definition.
(nuclear reactor)
'cleric astride american' is the wordplay.
'cleric' becomes 'rector' (type of clergyman).
'astride' means one lot of letters goes inside another.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'rector' placed around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "Apparatus for production of nuclear energy" , "Provider of nuclear power" , "Nuclear installation" , "Nuclear power generator" , "one hosting moderator?" .)