Pertaining to eight (5)
I believe the answer is:
octal
'pertaining to eight' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for octal that I've seen before include "Working in base 8" , "Pertaining to a number system with 8 as its base" , "Of a number system with base eight" , "that must omit a couple of digits" , "Using 8 as base (maths)" .)