If the button used for the belt buckle head is the same as someone else’s patented button, and you apply for a patent for the entire belt buckle head, the patent can be authorized.
But because your patent also includes someone else’s prior patent, when you use your patent to make money, you need to obtain the consent of the patentee of the prior button patent (if his patent is still valid). Otherwise it is an infringement.
Of course, patent rights need to be asserted by the patentee. In other words, although you have constituted an infringement, if the other party does not sue you, you will not suffer any loss.