They betray their principles and scratch one off the list in South Korean city (8)

'they' is the definition.
Both the answer and definition are plural nouns.
Maybe there's an association between them I don't understand?

'betray their principles and scratch one off the list in south korean city' is the wordplay.
I cannot quite understand how this works, but
'betray' could be 'sell' (synonyms) and 'sell' is found within the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'south' could be 's' (abbreviation) and 's' is present in the answer.
A single letter 'u' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?