Weak part of cliff railway (5)
I believe the answer is:
frail
'weak' is the definition.
(I know that weak can be written as frail)
'part of cliff railway' is the wordplay.
'part of' says the answer is hidden in the clue.
'FRAIL' is hidden in the letters of 'cliff railway'.
(Other definitions for frail that I've seen before include "Weak and infirm" , "Slight" , "isn't sturdy" , "Decrepit" , "rush" .)