Reportedly is familiar with disreputable bar — it will get you down fast! (4,4)
I believe the answer is:
nose dive
'it will get you down fast' is the definition.
(downward plunge in a plane)
'reportedly is familiar with disreputable bar' is the wordplay.
'reportedly' indicates a 'sounds like' (homophone) clue.
'is familiar with' becomes 'knows' (to know something is to be familiar with it).
'disreputable bar' becomes 'dive'.
'knows' is a homophone of 'nose'.
'nose'+'dive'='NOSE-DIVE'
(Other definitions for nose dive that I've seen before include "Sudden and dramatic deterioration" , "Swiftly drop" , "Headlong descent" , "Steep vertical plunge" , "Sudden headlong plunge" .)