Income, sad to say, taken back by railway (6)
I believe the answer is:
salary
'income' is the definition.
(I've seen this before)
'sad to say taken back by railway' is the wordplay.
'sad to say' becomes 'alas' ('alas' is said before reporting something sad).
'taken back' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation for railway).
'alas' in reverse letter order is 'sala'.
'sala'+'ry'='SALARY'
(Other definitions for salary that I've seen before include "Worker's pay" , "Stipend" , "Monthly pay" , "Regular earnings" , "Regular income from employment" .)