The crossword solution for the term ONE-THIRD OF 'TRI-' is MONO, which is a four letter word. Mono is derived from the Greek word monos, meaning 'ALONE' or 'SINGLE'. It is used to describe something that is made up of one part or element, such as a mono sound or a monochrome painting. In this case, it is used to refer to one-third of the prefix 'tri-', which means three. Therefore, the answer to the crossword clue is MONO, which means one-third of 'tri-'.