Either form is acceptable. (The history of new words formed by combining two existing whole words shows that a hyphen is often used for clarity when the term is first coined; as time passes and the compound word becomes familiar, the hyphen tends to be dropped.)

I agree with Ruth. It's definitely a dictionary question. Collins includes "meatpacker" https://www.collinsdictionary.com/dictionary/english/meatpacker Merriam Webster gives "meat-packer" https://www.merriam-webster.com/dictionary/meatpacker?utm_campaign=sd&utm_medium=serp&utm_source=jsonld

My initial reaction would be "meat-packer", but neither are wrong. Frequently, use of hyphens differ due to regions, dialects (U.K./ Ireland v US, v Australia, etc), or colloquialism. But it is really (really) not something to worry about too much. People who speak English as their first language will often mix these up or not be consistent. You will be understood, no matter which you choose.