Adding a 120v branch to a 4 wire 240v?
Most 240v spas have a 4 wire feed, so it is physically possible. The process for doing it would depend on the configuration of the spa disconnect, a picture of the disconnect would be good.
To legally do it where I live the breaker and wire in your panel that feeds the hot tub disconnect would need have capacity of 15A greater than the ampacity shown on the spa label.