Compounds perish, form does not. Because of this, forms, not particulars, are objects of knowledge. We would need some special idiosyncratic name to define them that picked out them alone. But the names we use are general and apply to more than one thing. Actually we cannot define anything that is unique or particular, even the sun and the moon.
Substance is then a sort of cause or reason why.
This can be the final or the efficient cause. But in a composite thing, we wonder how the whole gets its unity. Both have the same elements, yet they are different. The syllable is something more, but it's added to the elements not as another element, but as a cause. This is the substance or primary cause of being.