Drop steeply (6)
I believe the answer is:
plunge
'drop steeply' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for plunge that I've seen before include "Thrust or dive into" , "Take the . . . . . . - dive right in" , "Dive quickly" , "Dive or swoop" , "Fall suddenly and uncontrollably" .)