Where an adder slides to and fro? (6)
I believe the answer is:
abacus
'where an adder slides to and fro?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "You could count on this once" , "It will support the architrave" , "one counts on it" , "An ancient counting frame with beads" , "What was counted on" .)