The word "一大早" is formed by combining the character "一" (one), which here denotes a singular point in time, with "早" (early), creating the base phrase "一早" meaning early morning. The character "大" (big) is prefixed to "一早" as an intensifier, amplifying the sense of earliness to convey "very early in the morning". Thus, the three characters sequentially build a composite meaning where "大" modifies "一早" to emphasize an earlier time than "一早" alone.