Extracting power from unstable isotope is superfluous (6)
I believe the answer is:
otiose
'superfluous' is the definition.
('otiose' can be a synonym of 'superfluous')
I can't explain the remainder of the clue.
(Other definitions for otiose that I've seen before include "Disinclined to work or exert oneself" , "Indolent or superfluous" , "Serving no practical purpose" , "Serving no good purpose" , "Lazy, idle" .)