The peek hints should point out that "l'una" means one, as in "one o'clock". "l'/la" and "una" mean nothing here when explained separately.
Confusing because of the article. One would have to know that time is meant.
Do you mean, can circa mean anything else than about time measurement? That is a good question to which I would like to see an answer. Thank you for asking it.
Circa means 'around' or 'about' (= approximately). It doesn't just relate to time. In the previous level we had the example "Cucino con circa un litro d'acqua - I cook with about a litre of water" .... Hope this helps!