For British Rail to accept one was a blunder (5)

'blunder' is the definition.
(I know that blunder can be written as boner)

'for british rail to accept one was a' is the wordplay.
I cannot really see how this works, but
'british' could be 'b' (abbreviation e.g. in 'BBC') and 'b' is found within the answer.
'a' could be 'one' and 'one' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?