Apology for shameful start on gold railway (5)
I believe the answer is:
sorry
'apology' is the definition.
(I have seen 'Common apology ' mean 'sorry' so perhaps 'apology' could also mean 'sorry')
'shameful start on gold railway' is the wordplay.
'start on' indicates taking the first letters (I've seen 'starts on' mean this).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'railway' becomes 'ry' (abbreviation for railway).
The first letter of 'shameful' is 's'.
's'+'or'+'ry'='SORRY'
'for' acts as a link.
(Other definitions for sorry that I've seen before include "Penitent; regrettable" , "Feeling regret" , "My apologies!" , "Showing regret" , "Feeling penitence" .)