Unable to see student wearing tie (5)
I believe the answer is:
blind
'unable to see' is the definition.
(I've seen this before)
'student wearing tie' is the wordplay.
'student' becomes 'l'.
'wearing' means one lot of letters goes inside another.
'tie' becomes 'bind' ('bind' can be a synonym of 'tie'**).
'l' going within 'bind' is 'BLIND'.
(Other definitions for blind that I've seen before include "Like a bat" , "Failing to see" , "shade" , "Unable to observe" , "In the dark" .)