Vision certainly evident in octet (8)
I believe the answer is:
eyesight
'vision' is the definition.
(eyesight is a kind of vision)
'certainly evident in octet' is the wordplay.
'certainly' becomes 'yes' (I've seen this before).
'evident in' indicates putting letters inside.
'octet' becomes 'eight' ('eight' can be a synonym of 'octet').
'yes' put inside 'eight' is 'EYESIGHT'.
(Other definitions for eyesight that I've seen before include "Visual faculty" , "quality of viewing" , "Seeing ability" , "Power of vision" , "Seeing faculty" .)