When you are about to take victory, but fail to win near the end, especially when victory should be assured. This term is usually used after repeated occurrences.
"Innumerable chances came Parrott's way in the remaining frames but the 1991 world champion was gripped by "clinchers disease" - the inability to clinch winning positions."
He went on to lose this match, even though he was winning by 5-1 in a first to 6 match. His opponent took the next 6 frames in a row to win 6-5.