A “symbol of defeat” is referring to something that stands for defeat. Symbolism is different from metaphor. A metaphor is a comparison. For example, if a flag is described as a quilt, that would be using the flag as a metaphor. If something is described as a symbol of defeat, that means that whatever it is means defeat to someone. For example, let’s say the “symbol of defeat” was a wound.
John’s wound was a symbol of defeat, and he gave up the will to live.
This is just an example I made up. In this case, the sentence is telling you what the wound meant to John. It was more than just a wound. It meant that he was defeated, and he lost the will to fight.