A number holding British Rail to be dismal (6)
I believe the answer is:
sombre
'be dismal' is the definition.
(I've seen this before)
'a number holding british rail' is the wordplay.
'a number' becomes 'some' (I have seen 'Undefined number ' mean 'some' so perhaps 'number' could also mean 'some').
'holding' indicates putting letters inside.
'british rail' becomes 'br'.
'some' placed around 'br' is 'SOMBRE'.
'to' is the link.
(Other definitions for sombre that I've seen before include "Dull, gloomy" , "Gloomy and dismal" , "Funereal" , "Dark and gloomy of manner" , "Dark and dismal" .)