Sometime it happens that I want to answer a question. But at some point I want to save it for some reason and continue later when I have time or more arguments.
I would like to know how I can proceed with this in MSE? thanks
Drafts are automatically saved. But only one per type of post (question and answer) and this only for a limited period of time (a week).
Below the input window you will sometimes see a "Draft saved." Thus if you just want to interrupt your work for a few hours and you will not use the site in between this should be fine. Just wait a bit before closing the window.
For details see Allow questions to be saved as drafts prior to posting
Another option is, as mentioned in the comment, to use the sandbox on the meta site. Even if your answer there should be "cleared" you can always retrieve the revision later. A drawback is that your draft is visible to everybody then.
Still another option is to just save your stuff some other place. Copy-paste the text and save it somewhere. If you cannot or do not want to save it on a local device and do not have some space in some 'cloud,' you could improvise and just use a draft of an email.