Hello, my name is XXXXX XXXXX it is my pleasure to assist you with your question today. Are you an employee rather than self employed?
The clause itself is not illegal and can be implemented in a contract to determine the requirements of the job and what is expected of you. However, it is not as simple as having such a clause and the employer claiming they an dismiss you at any time if you do not meet the targets that are set within. Employment law dos provide you with certain rights, the main of which is protection against unfair dismissal, meaning that to fairly dismiss you an employer must show that there was a fair reason for doing so and also follow a fair procedure. So for example in the event of not reaching targets they would need to offer additional training, look at the targets and how realistic they are and so on. The issue is that such protection does not kick in until you have been employed by an employer for at least 2 years. So until then, you can be dismissed at any time and more or less for any reason, whether such a clause exists or not. But once you reach the 2 year threshold, even with such a clause in place the employer will need to follow a fair procedure and cannot just rely on the contract to dismiss you
There is nothing stopping you from trying to negotiate a rewording to make it clearer, but the employer does not have to agree to this. As mentioned you have no unfair dismissal protection yet so they could easily say you either take it as it is or leave it, so you could ask for the rewording but if they refuse don't push them too much as they could easily terminate your employment if they believe you are being difficult
you are most welcome, all the best