what!? no, don't need to add it as static (unless, of course, you want many objects from the class to use the same clock). are you used to program in Python or something? it's different in C++
just move the clock outside the funcion, but still inside the class, something like this:
class Rect {
sf::Clock cl1;
public:
Rect() {}
void RectOnClick(int Mousex, int Mousey) {
Lemon_Milk.loadFromFile("C:/Users/Felipe Palermo/source/repos/sfmllearn/sfmllearn/LEMONMILK-Bold.otf");
static int contagem = 1;
bool LockClick = false;
if (sf::Mouse::isButtonPressed(sf::Mouse::Left) && LockClick == false) {
//the rest of the code is the same