The term 'ONE CAUSING ESTRANGEMENT' can be summarized as an 'alienator' (9 letters). An alienator is someone who causes estrangement, or a separation, between two people. This could be done through intentional actions, such as spreading rumors or gossip, or through unintentional actions, such as neglecting to communicate with someone. Alienators can cause a rift between two people, leading to a lack of trust and communication.