The word slenderises 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: 14
Highest Scoring Words With Friends™ Plays In The Letters slenderises
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Seven Letter Word Alert: (30 words)
deniers, desires, diesels, dresses, eldress, endless, lenders, lessees, lessens, linseed, needers, needier, needler, needles, redline, redness, relined, relines, reseeds, resends, resides, seeders, seedier, senders, sessile, sidlers, sinless, slender, sliders, sneered