Full Idea
The claim that 'proté ousia' is substance is a particular answer to 'What is proté ousia?', so 'substance' is not what it means. The Latin 'substantia' translates Aristotle's 'to hupokeimenon' ('that which lies under').
Gist of Idea
In Aristotle, 'proté ousia' is 'primary being', and 'to hupokeimenon' is 'that which lies under' (or 'substance')
Source
report of Aristotle (Metaphysics [c.324 BCE], ousia) by Vassilis Politis - Aristotle and the Metaphysics 1.3
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.12
A Reaction
It seems that in 'Categories' Aristotle identified 'primary being' with 'that which lies under', but the notion of 'essence' comes into the picture in 'Metaphysics'. Big problems of textual exegesis.