"Be one back" can be described as a phrase used to indicate the trailing or losing position of someone or something. Antonyms for this phrase could include "ahead," "leading," or "in front," which all denote a position of advantage or success. Another possible antonym could be "tied," which suggests an equal or even position with competitors. It's important to note that the meaning of "be one back" can vary depending on context, so antonyms may also differ accordingly. Regardless, understanding antonyms for this phrase can help convey a more nuanced understanding of a situation or competition.