Professor on board with Eccles beginning to take plunge (7)
I believe the answer is:
plummet
'plunge' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for plummet that I've seen before include "Fall like a stone" , "fall sharply" , "Fall rapidly" , "Nose-dive" , "Fall at high speed" .)