The term 'winning length in a horse race' refers to the distance between the winning horse and the second-place horse at the end of a race. This term can be summarized by the crossword solution 'STRIDE, ' which is a six-letter word meaning 'a long step or leap.' In horse racing, a STRIDE is the length of a horse's gait, which is determined by the length of its legs. Therefore, the winning length in a horse race is the length of the STRIDE of the winning horse.