Possible Clues:

• I've come across hundreds on the way down
• Squalid joint
• Type of starter band gig
• See 17-Across
• Compete at a pool in the Olympics
• Squalid place
• Submerge
• It's a comedown, though some evidence points the other way
• Is it hundreds I have to get to take the plunge?
• What footballers might do illegitimately in dodgy club