Bovine spongiform encephalopathy (3,3,7)
I believe the answer is:
mad cow disease
'bovine spongiform encephalopathy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mad cow disease that I've seen before include "BSE (colloq.)" , "killer" , "Bovine spongiform encephalopathy" , "Beastly ailment" .)