Yes, you should, but you should keep it short. You can always back announce it once you're done if you find yourself noodling onstage and it develops into the tune.

I used to introduce one song I do as being learned at my father's knee... we were too poor to afford paper so he wrote it on his knee (OK so I stole it from the Two Ronnies' alter egos 'Jehosophat and Jones' but it's still funny)!

LTS