Top professionals turned up carrying Caesar's first ballista (8)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

reverse NO I PROS (top professionals) and insert the C from Caesar. Apparently the Scorpio(n) and Ballista were both Roman artillery pieces, but far from being the same thing; I had to learn from the web https://allthedifferences.com › ballista-vs-scorpion