Possible Clues:

• Sign back of cheque
• Back ' 'obson's choice'?
• Does it result in a demonstration of streamers?
• New party's outgoing language needs support
• Tip an originally overlooked runner to become champion
• Sign the back
• Sore end (anag.)
• Say you approve of kinky red ones
• Finish with unusually sore back
• Approve — support