Possible Clues:

• Block to progress
• A barrier to progress for some races
• Let jazz fans into joint, endlessly upset
• Something to be negotiated
• It hinders or prevents advance
• Need to get over this at meeting ¿ that's the catch!
• Block
• Let jazzmen into joint, mostly, on reflection
• A stumbling block to Basle, disunited about a century
• "Let" — for the first lob ace's sent out