Drop down, plunge (7)
I believe the answer is:
plummet
I believe this is a double definition.
'drop down' is the first definition.
(I've seen this before)
'plunge' is the second definition.
(synonyms)
(Other definitions for plummet that I've seen before include "Descend" , "Fall like a stone" , "Fall at high speed" , "Fall rapidly" , "Drop straight down" .)