In base eight (5)
I believe the answer is:
octal
'in base eight' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for octal that I've seen before include "Working in base 8" , "with unusual base" , "Relating to the number 8" , "Pertaining to a number system with 8 as its base" , "(Of maths) in base eight" .)