Some characters concoct alibis based on the number eight (5)
I believe the answer is:
octal
'based on the number eight' is the definition.
(I've seen this in another clue)
'some characters concoct alibis' is the wordplay.
'some characters' says the answer is hidden in the clue (I've seen 'some characters of' mean this (some letters or characters from the clue)).
'OCTAL' can be found hidden inside 'concoct alibis'.
(Other definitions for octal that I've seen before include "that must omit a couple of digits" , "(Of numbers) in base 8" , "type of base" , "based on eights" , "Using 8 as base (maths)" .)