I could just say what the sub is, but it is best if you read this article which will explain what jicama is and then you can pick your own substitute or use the one provided (or google it and you will find many more substitutes !) maybe on that suits your pallet best.
What is Jicama?
Can you explain how to use "jicama" — I think I know it is a fruit, but, I'm not really sure. Are there any other uses for it other than just eating like a fruit?
Jicama, also called the Mexican potato, is a large root vegetable with a thin brown skin and a white crunchy flesh. It is becoming more and more popular in Asia and the US, but hasn’t been widely used outside Mexico or parts of South America until recently. In texture, and to a lesser extent in taste, it is very similar to the water chestnut.
It is peeled and eaten raw or cooked, and has a taste that is described as slightly sweet and nutty. You might get a better sense of the taste by considering what some cookbook authors recommend as substitutes if you can’t find jicama — uncooked in a salad, the best substitution might be a firm, not-too-sweet apple. In a cooked dish, go the water-chestnut route.
There are an awful lot of recipes available that use jicama — either in a starring role or as a humble contributor — among them, lots of salads, slaws, salsas, relishes, and fish dishes.