twelvetone
[...] I'm just clapping and singing the song in my head. [...]

If you use a microphone, Shift-M is more useful to me than with a metronome. People will naturally try to synch to something they hear, so the metronome can derail some rather than be a guide (especially for creation and you engage a metronome off by 30 bpm).
 
If you are clapping/singing/humming into a microphone, ever better. When you have a natural feel for it, capture it (no metronome at all), and use shift-M (if you were singing/humming too, you also captured key). You can always fine-tune both of these later on, but that is very quick and easy.