Went slowly? I have to fall rapidly (8)
I believe the answer is:
nosedive
'fall rapidly' is the definition.
(I know that to nosedive is to apace fall)
'went slowly? i have' is the wordplay.
'went slowly?' becomes 'nosed' (nose can mean to move forward slowly).
'i have' becomes 'I've' (contraction of 'I have').
'nosed'+'ive'='NOSEDIVE'
'to' acts as a link.
(Other definitions for nosedive that I've seen before include "plummet" , "drop sharply" , "headlong plunge" , "sudden drop" , "steep fall" .)