A metaphor is an example of figurative language, when one thing is taken and compared to something completely different in order that we might see the similarities. Metaphors are very direct forms of figurative language, as one thing is said to be something else. An example would be: "The sea was a raging army that morning." This metaphor compares the sea to a ranging army by saying it is a raging army. Clearly, that morning, thanks to the wind and the weather, the sea was bobbing up and down quite fiercely and therefore resembled a mass of troops angrily making their way towards the land.
To come up with a metaphor for Clover you need to identify Clover's qualities, and then pick one or two of those and see what comparisons you can think of. One of Clover's defining characteristics is her loyalty to the revolution. Note the following quote:
Their most faithful disciples were the two cart-horses, Boxer and Clover. These two had great difficulty in thinking anything out for themselves, but having once accepted the pigs as their teachers they absorbed everything that they were told, and passed it on to the other animals by simple arguments. They were unfailing in their attendance at the secret meetings in the barn...
So, one quality of Clover's that you can identify is her zeal for the revolutionary cause. An appropriate metaphor you could use to describe Clover therefore might be something like:
Clover's zeal for the revolution was a fire burning constantly no matter how strong the wind and the rain outside.
Such a metaphor describes Clover's feelings about the revolution and captures the qualities of how strong those feelings are by comparing it to a fire. I hope this helps you identify what a metaphor is and how to construct one. Good luck!