Anything you do in your own time is generally unenforceable
With the important caveat that your employment contract may include clauses that give them rights over that stuff anyway, and even if they're unenforceable you could still end up having to fight in court over it.
Definitely something to keep in mind when reading the contract over, and ideally get a lawyer to take a look. It can be expensive, but weigh that expense against the potential expense of what would happen if you get screwed over.