Nuclear reactor (7)
I believe the answer is:
breeder
'nuclear reactor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for breeder that I've seen before include "Red beer (anag)" , "Type of nuclear reactor - a rabbit is a fast one!" , "One producing offspring" , "Siring horse" , "Stud owner" .)