Vision certainly present in a number (8)
I believe the answer is:
eyesight
'vision' is the definition.
(eyesight is your vision)
'certainly present in a number' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'present in' means one lot of letters goes inside another.
'a number' becomes 'eight' (eight is a kind of number).
'yes' going inside 'eight' is 'EYESIGHT'.
(Other definitions for eyesight that I've seen before include "Showing sense" , "Visual faculty" , "Power of vision" , "Power of seeing" , "Yes, eight required for vision" .)