The Apple app Zarf has an excellent built in clock for competitive Scrabble games, however, if you don't have Zarf, here are two excellent alternatives:
Wali Timer for android, developed by Wali Muhammad Khubaib
ChessClock for iOS, developed by Chess.com
Wali Timer for android, developed by Wali Muhammad Khubaib
ChessClock for iOS, developed by Chess.com