1. one
残りの問題は一つだな。There is one problem left.
2. for one thing
often used in itemized lists
4. (not) even
with a verb in negative form
見る限り、やはり傷一つない。As far as I can see, there is still not a single scratch.
5. just (e.g. "just try it")
6. some kind of; one type of
そして、俺は一つの可能性を考えていた。And I was considering one possibility.