I think it was written specifically for the episode.
Every mention of it that I can find online only refers to it as an “extract” and almost always includes a mention of Doctor Who. I’m also unable to find any mention of Eric Ritchie, poet or otherwise, that doesn’t include a mention of this poem.
As further evidence that this was probably a made-up poem, the Google Trends for “Thoughts on a Clock” and “Eric Ritchie” are zero until December 2013, when the two phrases suddenly spiked in popularity. If it was a real poem, I’d expect it to show up somewhere before it was mentioned in a Doctor Who episode.
(If nothing else, the Doctor Who writers would have Googled it, right? I mean, I know this is a universe with disappearing blue boxes and immortal aliens, but the idea that they use Bing really stretches plausibility.)