In UnNethack, how to wake just ONE monster?

Well, to draw an answer to this question from the comment you've already agreed with, you probably can't:

I don't think you can. (Where would be the fun in that? :P) You could just leave and come back when you're more prepared. – Doorknob Jun 21 '15 at 23:30

You have a point. It's one of the things that makes UnNethack more...interesting than vanilla Nethack. – SMBiggs Jun 22 '15 at 15:02