Nuclear reactor accident (8)
I believe the answer is:
meltdown
'nuclear reactor accident' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for meltdown that I've seen before include "Disastrous overheating of core of nuclear reactor" , "Sudden disastrous failure (of nuclear reactor)" , "Total disaster" , "Nuclear reactor accident" , "Disastrous event like overheating of nuclear reactor as in Chernobyl" .)