The word carthorse 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™ Play In The Letters carthorse
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Seven Letter Word Alert: (26 words)
archers, carrots, carters, charter, chaster, coarser, coaster, crasher, craters, creator, earshot, hoarser, reactor, rechart, recoats, rectors, reroast, rhetors, roaches, roaster, shorter, torches, tracers, trasher, trocars, trochar