British Rail now ruined by ex-PM (5)
I believe the answer is:
brown
'ex-pm' is the definition.
(former UK PM Gordon Brown)
'british rail now ruined by' is the wordplay.
'british rail' becomes 'br'.
'ruined' indicates an anagram.
'by' says to put letters next to each other.
'now' anagrammed gives 'own'.
'br'+'own'='BROWN'
(Other definitions for brown that I've seen before include "Capability" , "It is used for wrapping" , "After half portion of bran, have" , "Colour (at No 11?)" , "Shade" .)