之一
zhī yī · HSK 4
one of
之一 = 之 (of) + 一 (one); literally 'of one' = one of (them)
之 is a classical possessive/connector particle (like 的), and 一 means 'one'. So 之一 tacks onto a group to pick out 'one of' it — e.g. 朋友之一 = one of my friends.
之 — foot 止 above ground line 一
Originally a pictograph of a foot (止) stepping forward from a baseline, meaning 'to go' or 'to proceed'. Over time the concrete 'going' sense faded, and 之 was borrowed as a grammatical particle marking possession ('of') or as a pronoun ('it'), which became its dominant use in Classical Chinese.
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'