Learn Swedish: Vi äter lunch klockan tolv. - We eat lunch at twelve o'clock.

It is much more natural to use äter (eat) for meals in Swedish. While English speakers frequently say "have lunch," saying vi har lunch in Swedish sounds more like you physically possess the food in your hands or bag, rather than the action of sitting down to eat it.

What happens to the word order if I put the time at the beginning of the sentence?

Swedish is a "V2" language, which means the verb must be the second piece of information in the sentence. If you move the time to the front, the verb äter has to come before the subject vi. The sentence becomes: Klockan tolv äter vi lunch.

Is klockan only used as a grammar word to tell time?

No, klockan literally translates to "the clock" or "the watch" (it is the noun en klocka with the "the" ending -n). When you put it in front of a number it acts like "o'clock," but it is also just the everyday word for a physical clock.