what is Clover thinking as she sees her husband carted off to his death?

No, her husband is not Old Major. He was a Pig that dies early in the novel. Clover watched Boxer being taken to the meat grinders. She yelled, bring to warn Boxer,

"Boxer!" cried Clover in a terrible voice. "Boxer! Get out! Get out

quickly! They're taking you to your death!"

All the animals took up the cry of "Get out, Boxer, get out!" But the van

was already gathering speed and drawing away from them.

Perhaps Boxer might have heard them but he was much too weak to do anything.