Source of power is ambiguous, following Nationalist uprising (7)
I believe the answer is:
Here is my best explanation:
'source of power' is the definition.
'nuclear' can be an answer for 'power' (nuclear power is a kind of power). I am not sure about the 'source of' bit.
I don't understand how the rest of the clue works.
(Other definitions for nuclear that I've seen before include "winter, sort of", "These weapons release atomic energy", "Sort of energy", "sort of family", "With dangerous weapons", "Relating to fission or fusion", "and 6 Down: Power producer", "Clue ran oddly to atomic", "Possessing atomic weapons" and "Using atomic energy".)