Weak part of cliff railway (5)
I believe the answer is:
frail
'weak' is the definition.
(similar in meaning)
'part of cliff railway' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'FRAIL' is hidden within 'cliff railway'.
(Other definitions for frail that I've seen before include "Weak and infirm" , "Slight" , "isn't sturdy" , "Decrepit" , "rush" .)