It looks like some sort of nasty abscess. She also has bumblefoot. I have zero experience with abscesses but this one may need to be lanced or something. But don’t quote me on that @Wyorp Rock
Can you post more pictures and give us some more info? Is the growth hot? Is it hard or squishy? Is this bird in a run or free range? Does she live with many other chickens or poultry?