Ability to see eight? Yes, surprisingly (8)
I believe the answer is:
eyesight
'ability to see' is the definition.
(I've seen this before)
'eight? yes surprisingly' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'eight'+'yes'='eightyes'
'eightyes' anagrammed gives 'EYESIGHT'.
(Other definitions for eyesight that I've seen before include "Faculty of vision" , "Purpose of certain organs" , "quality of viewing" , "Visual faculty" , "Showing sense" .)