Use GarageBand's metronome for practice
Well, you could just start a recording every time you practice and delete it at the end. Not very practical probably.

Alternatively you could download Metronome which "is designed with the practicing musician in mind which means that it's fast and easy to use. Nearly all of Metronome's functions and settings can be changed by using a single key." Actually there are probably a lot of other metronome applications, this was just the first I found in Google.