It's just a fragment of a sentence.

한순간 = momentarily, for a moment.

거짓이 된 = attributive adjective form of 거짓이 되다 = become a lie/falsehood.

So 한순간 거짓이 된 = which has become a lie for a moment.

The meaning is unclear without the noun coming after it and the broader context, but it probably means that someone's word has been rendered false by some contradicting fact.