The word defervescences 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: 29
Highest Scoring Words With Friends™ Play In The Letters defervescences
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Seven Letter Word Alert: (28 words)
censers, decenes, decrees, defence, defense, deserve, essence, feeders, fencers, fenders, fevered, needers, recedes, redness, refeeds, refence, reseeds, resends, screeds, screens, seceder, secedes, secerns, seeders, senders, severed, sneered, veneers