Faculty certainly entertained by cardinal (8)
I believe the answer is:
eyesight
'faculty' is the definition.
(I know that eyesight is a type of sight)
'certainly entertained by cardinal' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'entertained by' means one lot of letters goes inside another (inserted letters are hosted or entertained).
'cardinal' becomes 'eight' (similar in meaning).
'yes' placed into 'eight' is 'EYESIGHT'.
(Other definitions for eyesight that I've seen before include "Seeing ability" , "Requirement to look" , "quality of viewing" , "Yes, eight required for vision" , "View" .)