Unhealthy part of rail link (3)
I believe the answer is:
ill
'unhealthy' is the definition.
(associated in meaning)
'part of rail link' is the wordplay.
'part of' indicates a hidden word.
'ILL' is hidden within 'rail link'.
(Other definitions for ill that I've seen before include "Badly - in need of medical care" , "Badly - in need of treatment" , "Afflicted" , "In a bad way" , "Badly diminished" .)