Railways cut on a regular basis, regrettably (4)
I believe the answer is:
alas
'regrettably' is the definition.
(I've seen this before)
'railways cut on a' is the wordplay.
'railways cut' becomes 'las' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'las' after 'a' is 'ALAS'.
'regular basis' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for alas that I've seen before include "Woeful expression" , "Lackaday" , "Regrettably, unfortunately" , "Proper, respectable" , "By bad luck" .)