The secret fruit is a random showing fruit that will appear on your screen.
You have to slice it for the achievment to pop.
The fruit is actually a Dragon Fruit, and when it comes on the screen you can't miss it. It's shinier, and much larger than the other fruits. And it looks like a Dragon Fruit (yes, a real fruit - it's red, with green bits on it).
I've only ever seen one, and it was in a game of Classic. (It seems it will only spawn in Classic)
It popped very early in the match, so I think its just random. If you guys found it in other modes let me know and I'll update the solution.