Express sadness over onset of tunnel vision (5)
I believe the answer is:
sight
'vision' is the definition.
('sight' can be a synonym of 'vision')
'express sadness over onset of tunnel' is the wordplay.
'express sadness' becomes 'sigh'.
'over' says to put letters next to each other.
'onset of' says to take the initial letters.
The initial letter of 'tunnel' is 't'.
'sigh'+'t'='SIGHT'
(Other definitions for sight that I've seen before include "Looking" , "Field of vision" , "Show" , "Glimpse" , "Power of vision" .)