I purchased an Acropolis and Slopes combo ticket and selected a time slot for entry into the Acropolis. I want to see the Ancient Agora earlier in the day before the Acropolis. I was under the impression that the Agora also required a timed entry, so how do I get that using this combo ticket?
Your purchased combo ticket does not cover entry to the Agora; you will need to buy an entry ticket for it as well. You can do this on the Hellenic Heritage website: https://hhticket.gr/tap_b2c_new/english/tap.exe?PM=P1P&place=000000004. Even though the site states you need a timed entry ticket, I understand that if you simply show up at the ticket stand there you can purchase your entry then, because the Agora doesn't attract the crushingly high number of tourists as the Acropolis does. There is little to no shade on the Acropolis or the Slopes, so wear a hat if you go any time after 10AM. Also something to note: wear sandals or shoes that do not slip on polished stone; it's incredibly slippery all over the site (even I have fallen there as recently as a couple of weeks ago, and I've been numerous times).
Due to the high demand, the "Acropolis & Slopes" is the only site where you must arrive at the time slot you reserved.
So far, the Ancient Agora is much quieter, and you can go there even if your arrival time doesn't match your booked time slot.
PS: You're talking about a combo ticket. The combo ticket that allowed you to visit 7 sites for €30 has been discontinued since April 1, 2025.
Since that date, you must purchase a separate ticket for each site.
eskee provided the link above.