Yes, rat is a Scrabble word!
...and is worth 3 points in Scrabble, and 3 points in Words with Friends
rat
Verb
1. Desert one's party or group of friends, for example, for one's personal advantage
2. Employ scabs or strike breakers in
3. Take the place of work of someone on strike
4. Give (hair) the appearance of being fuller by using a rat
5. Catch rats, especially with dogs
6. Give away information about somebody
Noun
1. Any of various long-tailed rodents similar to but larger than a mouse
2. Someone who works (or provides workers) during a strike
3. A person who is deemed to be despicable or contemptible
4. One who reveals confidential information in return for money
5. A pad (usually made of hair) worn as part of a woman's coiffure
What Scrabble words can I make with the letters in rat?
3 Letter Words
2 Letter Words