Not even once speech was curtailed on a numerical system (5)
I believe the answer is:
octal
'system' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for octal that I've seen before include "Pertaining to a number system with 8 as its base" , "Working in base 8" , "Of a number system with base eight" , "based on eights" , "(Of maths) in base eight" .)