Energy released in nuclear fission (6)
I believe the answer is:
atomic
'energy released in nuclear fission' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for atomic that I've seen before include "Very little" , "Sort of bomb, number, or power" , "Realting to nuclear energy" , "Extremely small-scale" , "of a small unit" .)