Sudden headlong plunge (4,4)
I believe the answer is:
nose dive
'sudden headlong plunge' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nose dive that I've seen before include "Plunge head first" , "Sudden headlong plunge" , "Sudden and dramatic deterioration" , "Steep descent" , "Steep downward plunge by aircraft" .)