Possible Clues:

• Is a half-century able to get dug
• Privy to a student's course?
• Suez or Panama
• Is a half-century able to hold 3 down?
• Can there be water there before the end of 19 across?
• Passage that's sexy censor finally cut
• Locks are the key to its existence
• Turn for the better tomorrow?
• Toll site
• Panama or Suez, for example