Complex sentence: A claim in unjust enrichment requires benefits that have been obtained by one person to be given up if it would be "unjust" to retain, because no other valid legal transaction is present, such as a contract, trust, gift or estoppel.
Split into simpler sentences: A claim in unjust enrichment requires benefits that have been obtained by someone to be given up if it would be "unjust" to retain them. The enrichment must be "unjust" if no valid legal transaction is present, such as a contract, trust, gift or estoppel.