The ancients were counting on this (6)
I believe the answer is:
abacus
'counting on this' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "calculator needed" , "Mechanical calculator" , "Beaded counting frame" , "one counts on it" , "Very early calculator" .)