Temporomandibular Joint (TMJ) Disorder: The patient's symptoms of jaw pain, especially under the left ear, pain when moving the jaw, and a history of jaw clicking are classic for TMJ disorder. The improvement with ibuprofen also supports this diagnosis, as it can help with the pain and inflammation associated with TMJ disorders. The patient's habit of teeth clenching is a common contributing factor to TMJ disorders.

Bruxism: Given the patient's history of teeth clenching, bruxism is a likely contributing factor to the jaw pain. Bruxism can lead to TMJ pain, jaw clicking, and pain, especially if the patient clenches their teeth at night.
Myofascial Pain Syndrome: This condition involves pain and inflammation of the muscles that control jaw function and can be related to teeth clenching and grinding. The patient's symptoms of jaw pain and improvement with ibuprofen could also fit this diagnosis.
Dental Issues: Although not directly mentioned, dental problems such as abscesses, impacted teeth, or gum disease could cause jaw pain. However, the absence of specific dental symptoms and the normal ear exams make this less likely.

Parotid Gland Tumors: Although rare, tumors of the parotid gland can cause jaw pain, especially if they are located near the TMJ. The mild cervical lymph node enlargement could be a red flag for a more serious condition, including parotid gland tumors.
Jaw Osteonecrosis: This condition, often associated with bisphosphonate use or radiation therapy, can cause jaw pain and is a critical diagnosis not to miss due to its potential for significant morbidity.
Lymphoma: The presence of mildly enlarged cervical lymph nodes, although nonspecific, warrants consideration of lymphoma, especially if other symptoms such as weight loss, fever, or night sweats were present (though not mentioned in this case).

Eagle Syndrome: This condition involves an elongated styloid process or calcified stylohyoid ligament, which can cause jaw pain, especially with movement. It's a rare condition but could be considered if more common causes are ruled out.
TMJ Synovitis: Inflammation of the synovial membrane within the TMJ can cause pain and dysfunction. While not as common as other TMJ disorders, it's a possible diagnosis in patients with persistent jaw pain and clicking.
Neoplastic Conditions of the Jaw: Various tumors, benign or malignant, can affect the jaw and cause pain. These are rare but important to consider in the differential diagnosis to ensure timely and appropriate management.