Knight's line in the old Southern Railway (3)
I believe the answer is:
sir
'knight's' is the definition.
(a knight has the title Sir)
'line in the old southern railway' is the wordplay.
I cannot really see how this works, but
'southern' could be 's' (abbreviation) and 's' is found in the answer.
'railway' could be 'r' (eg in Great Western Railway) and 'r' is found in the answer.
A single letter 'i' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sir that I've seen before include "as Winston Churchill was" , "Way to address gentleman" , "Title of knight or baronet" , "Knight's prefix" , "Title for knight or teacher" .)