Sudden drop (8)
(Other definitions for dip that I've seen before include "An informal swim or a depression" , "Dunk" , "Thick sauce; brief swim" , "baptism" , "Go down" .)
'sudden drop' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nosedive that I've seen before include "headlong plunge" , "fall precipitately" , "Take the plunge suddenly" , "drop sharply" , "sudden drop" .)