Translation:It is one o'clock.
Yes, just like English, Japanese has synonyms.
I'm not 100% about this, but I feel like 一瞬 is shorter than ひととき, since another possible translation for 一瞬 is "an instant".
In English, when we say "Please wait a moment", that "moment" is longer than when we say "In that moment, my life flashed before my eyes". In the latter, we can replace "moment" with "instant", but not in the former. That's the difference between 一瞬 and ひととき I think.
Japanese doesn't actually have participles. I think you meant "particle", but no, it's not necessary here.
Each particle has a specific role to play in a sentence. The way this sentence is set up literally means "is (=です) 1 o'clock (=一時)". But what is 1 o'clock? That's what you need a particle to mark for you. Usually. But lucky for us, Japanese lets you leave out the "what" and use the context to figure it out. So, if you leave out the "what", you don't need to mark anything, therefore no particle required.
All these o'clock sentences, DuoLingo keeps telling me o clock (with a space instead of an apostrophy) is a spelling error. o'clock developed in the old days as a quicker slur, so why can't o clock be considered a more convenient, still valid spelling? Everyone I know spells it with a space rather than an apostrophy in writing, and the end pronunciation is the same, so can we not be called out for spelling errors over it?