meaning in context - would you throw a light on the concept of the sentence?

In these clinical cases, as in other phobias, the more likely cause is a displacement of diffuse anxiety to an external focus which can be avoided.

I am wondering what this sentence could mean or what is the concept if it.

What is more, would you please in a more readily way show me what is meant by the word clinical cases?

And, really what does external focus mean there??

Extracted from the following; Psychopathology:

Any feed-back would be greatly appreciated

I cannot yet get the following:

the ... cause is a displacement of diffuse anxiety to an external focus which can be avoided.

UPDATED:Thanks. Yet, could anyone please show me the concept of the sentence through a vivd example?