If you're getting slipping, then this isn't just a worn chain, it's a worn cassette and chain.
If the "cassette is pretty clean" it's because you've cleaned it ans not/scarcely ridden it. Difficult to tell by inspection if a cassette is worn beyond use.
Absolutely no point in measuring the chain or scrabbling for esoteric possibilities: sticky freehub, sticky chain links.
Buy both and install. Just replacing the chain will make the slipping worse - and right across the cassette - because the (now worn) cassette has grown up with the chain and can cope with it, except on the sprockets you use most for putting power down.