Sudden drop seen with void when flying (8)
I believe the answer is:
nosedive
'sudden drop' is the definition.
(I've seen this before)
'seen with void when flying' is the wordplay.
'with' means one lot of letters go next to another.
'when flying' indicates an anagram (letters fly around).
'seen' put after 'void' is 'voidseen'.
'voidseen' is an anagram of 'NOSEDIVE'.
(Other definitions for nosedive that I've seen before include "plummet" , "sudden drop" , "Dramatic fall" , "(Make a) steep downward plunge" , "Take the plunge suddenly" .)