Dirty Den's rapid descent (4)
I believe the answer is:
dive
I believe this is a double definition.
'dirty den's' is the first definition.
(term for a dirty or disreputable bar)
'rapid descent' is the second definition.
(Other definitions for dive that I've seen before include "Plummet" , "Jump into pool" , "Low establishment" , "Plunge into a low haunt" , "Leap into water" .)