Pain under the left clavicle may be due to the following reasons: 1. local fasciitis or enlarged lymph nodes under the left clavicle, which can cause pain under the left clavicle. 2. cervical spondylosis, which can result in intervertebral disc compression of nerves, causing pain in the area where the nerves are located. 3. strain or degeneration of the neck, muscle strain can also cause pain under the left clavicle. 4. The pain under the left clavicle may also be due to inflammation of the lungs or tuberculosis, or even tumors of the lungs, which can cause local pain. 5. Problems of the left clavicle itself, such as tumors of the clavicle, tuberculosis of the clavicle, aseptic inflammation of the clavicle, or even inflammation of the muscles, fascia, and soft tissues under the left clavicle, or tumors, can cause local pain.