From Longman Dictionary of Contemporary Englishby a neckby a neckinformalLITTLE/NOT MUCH if a race, especially a horse race, is won by a neck, the winner is only a very short distance in front Our horse won by a neck. → neck
Examples from the Corpus
by a neck• The springing swan-bows, neck by neck, would slide homing into the sand-flats like silk.• But things have now improved and sales in comparable shops are currently ahead of last year, if only by a neck.• Although tying up, she battled on well to hold off Flying Speed by a neck.• Our horse won by a neck. 