Hi,
Swelling between the neck and clavicle could be due to trauma at the site, local infection including skin infection, Lymph node enlargement - the supraclavicular lymph nodes are here, Due to pus collection /
abscess formation at the site. It could be a solid or cystic lesion.
Do you have any other associated symptoms of pain, redness? Is it warm to touch? Do you have any fever? For how long have you noticed the swelling? Do you remember any trauma or injury to the site?