Nature's mutated when uranium's left behind (6)
I believe the answer is:
astern
'behind' is the definition.
(at the back or behind of a ship)
I don't understand how the remainder of the clue works.
(Other definitions for astern that I've seen before include "Aft" , "Towards rear of a vessel" , "Abaft" , "Behind at sea" , "Towards a ship's rear" .)