Let’s take a look at this paragraph I borrowed from the Stoic Ethics section of the Internet Encyclopedia of Philosophy:
The Stoics elaborated a detailed taxonomy of virtue, dividing virtue into four main types: wisdom, justice, courage, and moderation. Wisdom is subdivided into good sense, good calculation, quick-wittedness, discretion, and resourcefulness. Justice is subdivided into piety, honesty, equity, and fair dealing. Courage is subdivided into endurance, confidence, high-mindedness, cheerfulness, and industriousness. Moderation is subdivided into good discipline, seemliness, modesty, and self-control.
If I had a to guess, I’d say humility might be nested in the sub-virtue piety. When we think of piety we usually think of religious devotion. But in this context, I think it means reverence for the universe. And if you revere the whole universe, you pay it a deep respect. Basically, you inflate the importance of the universe and you deflate your importance relative to it. So you basically humble yourself before the Divine Whole of the Universe.
Another place where humility would be nested would be in modesty. Modesty means being moderate in one’s estimation of one’s abilities. So there appears to be two places where humility resides as a sub-virtue.