Trouble with the carriage at the end of the line (4)
I believe the answer is:
care
'trouble with' is the definition.
'carriage at the end of the line' is the wordplay.
'carriage' becomes 'car' (eg a train car or train carriage).
'at the end of' indicates one should take the final letters.
The final letter of 'line' is 'e'.
'car'+'e'='CARE'
'the' is the link.
(Other definitions for care that I've seen before include "Have concern or regard" , "Consideration" , "Serious attention and thought" , "Protective custody" , "Caution; keeping" .)