Translation:He started in the center of the page.
leathanaigh is the Irish for "pages". It is also the Irish for "of the page".
The "clue" is just a simple dictionary lookup. It has no way of knowing whether the word that you are looking up is a plural word or the genitive of a singular word. You were "mislead" by your inability to recognize that you have to use the genitive of a noun after i lár - "in the centre of".
The "hints" or "clues" aren't a shortcut to the correct answer - if you want to see the correct answer, click on Submit. As the singular definite article is used in an leathanaigh, it's obviously not plural "pages", but the hint should have been enough to remind you what the root of leathanaigh is.
Maybe an oddity - centre vs. center - I realize the first is English, the second, generally accepted as American. I notice sometimes when I spelling either of them in an exercise, it will be underlined as incorrect in the answer. In daily use I would use centre as a place/building , e.g. Arts and Culture Centre but center to mean the middle of something. Is there a consensus among other English speakers for similar usage?
Americans use "center" for both the "middle" and the "place/building" meanings. Places that follow "British English" spelling norms use "centre" for both the "middle" and the "place/building" meanings.
The concensus among English speakers is that you either use "center" or you use "centre", you don't mix and match.