slightly formal to be determined to prove that you are good at something, especially in a difficult situation.
It's a tough interview - you'll have to be on your mettle.
be on (one's) mettle
To be determined to succeed and thus prove one's worth, often in a difficult or unpleasant situation. I know my employees think I'm too young to be their supervisor, so I have to be on my mettle every day at the office.
be on your mettle
be ready or forced to prove your ability to cope well with a demanding situation.