Powerful agency about to take on a member of the cast (7)

I believe the answer is:

reactor

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But I can't explain it at all! Could you help me understand?

(Other definitions for reactor that I've seen before include "nuclear facility" , "Heart of a nuclear power station" , "Nuclear power plant" , "Nuclear power source" , "Producer of nuclear energy" .)

I've seen this clue in The Globe and Mail.

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