Iapetus impetus

[Paucle]

Iapetus impetus took me so long to get (almost a day) because I mis-brained it as an ana-pair, and I was playing with anagrams of all Saturn's major moons, then not so major, etc etc.
What's funny is that when I parsed that Staley was actually seeking a "one-letter," I didn't even have to review the list, I remembered considering Iapetus and saying to myself at the time, "That almost anagrams into impetus."

[Staley]

I do that (look for anagram instead of one-letter) all the time.