Source of power (7,6)
(Other definitions for nuclear energy that I've seen before include "Elementary source of heat and electricity" , "Result of fission or fusion - near Uncle Grey (anag)" .)
'source of power' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for steam that I've seen before include "Kettle emission" , "Driving force" , "Vapour from water at boiling point" , "Vaporous exhalation" , "Vapour; energy" .)