What caused this? Did you have trauma in the area? While unlikely, I would ensure there isn't a fracture of the zygomatic arch. More likely is some localized soft tissue necrosis from trauma. In that case, can fill the dent in with something. Ideally you will use your body's own tissue rather than an artificial material. Will also need to separate overlying skin from the deep tissue (subscision as mentioned above is one method) to allow the filler to be placed.One option is to harvest temporoparietal fascia and some temporalis fascia from the just in front of the ear, and through the same incision lift the skin off the deeper tissue and place the grafts.
Subcision followed by an implant such as eptfe will usually fix indentations quite easily under local and quickly in 1 treatment.