TK spoilers
Don't read this if you're still trying to figure out what common English word contains tk in the middle!
Shmuel and Mary Anne both quickly came up with the less-common loanword I was thinking of: latke. I think that's probably obvious to some people, but not to everyone. I only found it by doing a dictionary search for words containing tk. But the at-least-I-think-it's-more-common word I originally had in mind is pocketknife. Perhaps that's cheating a bit; some people no doubt spell that with a space or a hyphen in the middle. But MW10 spells it as a closed compound.
Now I really gotta go work.