Sudden plunge (4,4)
I believe the answer is:
nose dive
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sudden 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 "Steep plunge (by aircraft); sudden deterioration" , "Steep vertical plunge" , "Steep descent" , "Dramatic drop" , "Front-first fall" .)