Take the plunge in a disreputable place (4)
I believe the answer is:
I believe this clue is a double definition.
'take the plunge' is the first definition.
(I've seen this in another clue)
'a disreputable place' is the second definition.
(a disreputable bar or club)
'in' is the link.
(Other definitions for dive that I've seen before include "Plunge head first" , "Seedy bar" , "Plunge headfirst" , "Submarine captain's command" , "Plummet" .)