Not now that "41" has caught cold (4)

I believe the answer is:

once

I'm a little stuck... Click here to teach me more about this clue!

But I can't explain it at all! Could you help me understand?

(Other definitions for once that I've seen before include "Formerly; as soon as" , "not repeatedly" , "Quick examination" , "Previously" , "At some former time" .)

Want a hint initially instead of a full solution? Install my app