Fall rapidly (7)
I believe the answer is:
plummet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fall rapidly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for plummet that I've seen before include "Drop vertically and fast" , "Descend" , "Fall like a stone" , "Fall at high speed" , "Plunge headlong" .)