Tap Tempo

click on it repeatedly as you would tap with your feet following the beat of listening music (you can hit the space bar too, if tap button is under focus)

at least at the third time you tap it will give you an approximate tempo value (bpm) of the actual beating/tapping you're doing :)

otoh. if your audio clip is in kind of a perfect loop(able) clip length, then you might try Clip/Tempo Adjust... dialog there :)

cheers