Paddington's home page initially enrages rail union (4)
I believe the answer is:
peru
'paddington's home' is the definition.
(home of the fictional bear)
'page initially enrages rail union' is the wordplay.
'page' becomes 'p'.
'initially' indicates taking the first letters.
The first letters of 'enrages rail union' is 'eru'.
'p'+'eru'='PERU'
(Other definitions for peru that I've seen before include "Machu Picchu country" , "Homeland of Paddington Bear" , "South American state" , "S American country, capital Lima" , "Andes republic" .)