The word interregnum is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 19
Highest Scoring Words With Friends™ Play In The Letters interregnum
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Seven Letter Word Alert: (28 words)
eminent, genuine, germier, grinner, grunter, gumtree, inerter, integer, meeting, minuter, miterer, nitrene, regimen, reigner, reinter, renting, retimer, reunite, ringmen, runnier, runtier, runtime, teeming, treeing, trireme, turning, unmerge, uterine