Clover is an excellent choice for the character in this exercise. Remember it is she rather than the other animals in this strong allegory who identifies soonest what Napoleon and the other pigs are up to, and it is she who tries to get the other animals to be aware of what is really going on. Note her reactions after the slaughter of the hens and the sheep for crimes against Napoleon. As she gazes at Animal Farm, she thinks:
If she could have spoken her thoughts, it would have been to say that this was not what they had aimed at when they had set themselves years ago to work for the overthrow of the human race. These scenes of terror and slaughter were not what they had looked forward to on that night when old Major first stirred them to rebellion.
I think an appropriate extended metaphor you might like to choose for your task would therefore be something that starts well and with great hope, only to slide gradually into despair and blackness. This is of course Clover's own experience of Animal Farm, capturing the hope of the initial rebellion but also the slow decline as it becomes clearer and clearer precisely what is going on. Given the farming life that Clover has experienced, you might want to use the process of eating an apple, that starts off sweet but becomes more sour and more rotten as time goes by. Or perhaps you could use a crop that looks as if it is growing really well and will yield a great harvest, only to be gradually eaten away by bugs, disease and bad weather. The impression overall must be on the slow decline of a once hopeful dream. I hope this helps and good luck with your task.