Port where British Rail got established (5)
I believe the answer is:
brest
'port' is the definition.
(I've seen this before)
'british rail got established' is the wordplay.
'british rail' becomes 'br'.
'got' means one lot of letters go next to another.
'established' becomes 'est' (abbreviated used in shop signs).
'br'+'est'='BREST'
'where' acts as a link.
(Other definitions for brest that I've seen before include "Breton port" , "Naval base in Brittany" , "French warships here in Brittany" , "French Atlantic naval base" , "French port" .)