Having investigated, I have to plunge (8)
I believe the answer is:
nosedive
'to plunge' is the definition.
(nosediving is a kind of plunging)
I can't explain the rest of the clue.
(Other definitions for nosedive that I've seen before include "Take the plunge suddenly" , "plummet" , "fall precipitately" , "sudden drop" , "headlong plunge" .)