Family-based treatment (FBT) is an evidence-based treatment for adolescent eating disorders that incorporates many principles from family therapy. It uses the externalisation of anorexic thoughts and behaviours to separate the person from the anorexia nervosa (AN) through language and metaphor. Little is known about how clinicians understand, conceptualise and support families to externalise. Semi-structured interviews conducted with FBT-trained clinicians working in child and adolescent mental health services were analysed using thematic analysis. Three themes emerged: the clinician’s use of externalisation, the impact on family functioning and the barriers to externalisation. Externalisation can support a young person’s recovery from AN when used in conjunction with other therapeutic skills. Clinicians should be aware of potential barriers to the implementation of externalisation, such as problem awareness, age and duration of illness. Practitioner points Clinicians value externalisation as an important therapeutic technique within the FBT model, while acknowledging that the ‘ED as illness’ metaphor can challenge their own beliefs. Externalisation can improve family functioning, family communication and reducing conflict within relationships in families with AN. Lack of insight into the AN can present as a barrier to externalisation for a young person.