an extra
Examples
Don't sleep too much. If you sleep three hours less each night for a year, you will have an extra month and a half to succeed in.
Remember you are just an extra in everyone else's play.
I have deeply regretted that I did not proceed far enough at least to understand something of the great leading principles of mathematics, for men thus endowed seem to have an extra sense.
An extra thing is an extra concern.
Give an extra piece of cake to a stepchild.
One more good man on earth is better than an extra angel in heaven.