Possible Clues:

• Result of jumping the gun
• Reason for a track race do-over
• F?
• Gridiron infraction, and a hint to 17-, 25-, 37- and 49-Across
• Beginning to doubt?
• Invalid off
• Synthetic tree decoration with timer at first going too quickly
• This could disqualify athlete getting off faster -- last
• In the course of events, it may lead to total recall
• Unsuccessful attempt to begin a race