Took a nosedive (9)
'took a nosedive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fell that I've seen before include "Highland moor" , "northern mountain" , "Abated" , "Chop down" , "Came down - cut down" .)
(Other definitions for plummeted that I've seen before include "went swiftly to bed?" , "Dropped sharply" , "Fell rapidly" , "Fallen" .)