Ability to see positive response in crew (8)
I believe the answer is:
eyesight
'ability to see' is the definition.
(I've seen this before)
'positive response in crew' is the wordplay.
'positive response' becomes 'yes' (positive response to a question).
'in' is an insertion indicator.
'crew' becomes 'eight' (an eight-person rowing team).
'yes' placed into 'eight' is 'EYESIGHT'.
(Other definitions for eyesight that I've seen before include "Faculty of vision" , "Power of vision" , "Requirement to look" , "Visual faculty" , "What's needed to see" .)