Subway at first rates No.1 for sandwiches (5)
I believe the answer is:
metro
'subway' is the definition.
(I know that subway can be written as metro)
'first rates no 1 for sandwiches' is the wordplay.
'first rates' becomes 'r' (mst letter of 'rates').
'no m' becomes 'me' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' becomes 'to' (eg both can mean 'in the opinion of').
'sandwiches' is an insertion indicator (goes on either side like a sandwich).
'me'+'to'='meto'
'r' put inside 'meto' is 'METRO'.
'at' is the link.
Can you help me to learn more?
(Other definitions for metro that I've seen before include "Paris subway" , "Parisian rail system" , "Parisian underground system" , "Urban railway system" , "Parisian underground railway" .)