That's a basic C++ problem, nothing related to SFML, let alone to a binding. In the future, please use other platforms (C++ forums, StackOverflow) for this kind of question.
Declare the variable in the class without definining it, and initialize it in the constructor initializer list. Please don't just ask how and expect 1:1 code; do a bit of research. These are absolute basics that you must know when you want to use a library like SFML.