I think there is some confusion about what the task actually is. In my thinking a task is something that is done by a clerk. So you are able to create a task itself without knowing about a clerk.
Based on that task you can choose an appropriate clerk for it. The assignment of the task to the clerk can itself be wrapped to some other kind of task. So a common interface for choosing a clerk would be:
interface ClerkResolver {
String resolveClerk(Task task);
}
For implementing this kind of clerk resolver you can use the strategy pattern based on the actual type of the task for example.