Having investigated, I have to plunge (8)

I believe the answer is:

nosedive

I'm a little stuck... Click here to teach me more about this clue!

'to plunge' is the definition.
(I know that nosedive is a more specific form of the action dive)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for nosedive that I've seen before include "Take the plunge suddenly" , "plummet" , "fall precipitately" , "sudden drop" , "headlong plunge" .)

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