Nuclear accident (8)
I believe the answer is:
meltdown
'nuclear accident' is the definition.
(I know that meltdown is a type of nuclear accident)
This is all the clue.
(Other definitions for meltdown that I've seen before include "Disastrous collapse - nuclear accident" , "Sudden disastrous failure (of nuclear reactor)" , "Total disaster" , "Disastrous event as to overheated nuclear reactor" , "Serious nuclear accident" .)