Part of the eye that’s concerning can start to atrophy (6)
I believe the answer is:
retina
'part of the eye' is the definition.
('retina' is part of 'eye')
'concerning can start to atrophy' is the wordplay.
'concerning' becomes 're'.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'start to' says to take the initial letters.
The first letter of 'atrophy' is 'a'.
're'+'tin'+'a'='RETINA'
'that's' acts as a link.
(Other definitions for retina that I've seen before include "Membrane that can become detached" , "Light-receptive part of eye" , "Light-sensitive eye lining" , "Eye's membrane" , "Ocular membrane" .)