Save area covered by old rail service (3)
I believe the answer is:
bar
'save' is the definition.
(I've seen this before)
'area covered by old rail service' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'covered by' means one lot of letters goes inside another.
'old rail service' becomes 'br' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' put into 'br' is 'BAR'.
Can you help me to learn more?
(Other definitions for bar that I've seen before include "Rigid rod" , "Batten" , "Counter in 21" , "Obstacle" , "Counter; ban" .)