Sudden headlong plunge (4,4)
I believe the answer is:
nose dive
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sudden headlong plunge</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)