You could also create a "wrapper" method (getFirstElement) on the SourceObject:

public class SourceObject {
public ArrayList<ListObject> list = new ArrayList<ListObject>();

public SourceObject() {
    list.add(new ListObject());
}

public getFirstElement(){
    return list.get(0);
}

So in your initial mapping you can say:

@Mappings({
    @Mapping(target="details.forename", source="firstElement.details.forename"),
    @Mapping(target="details.surname", source="firstElement.details.surname"),
})
DestinationObject toDestination(SourceObject source);