Former railway food in the buffet? (6)
I believe the answer is:
pastry
'food in the buffet?' is the definition.
(food which might be part of a buffet)
'former railway' is the wordplay.
'former' becomes 'past' (similar in meaning).
'railway' becomes 'ry' (abbreviation for railway).
'past'+'ry'='PASTRY'
(Other definitions for pastry that I've seen before include "'It may be puff, shortcrust and flakey (6)'" , "Pie base" , "'Dough of flour, fat and water (6)'" , "Eg, pie crust" , "Dough made for pies, etc." .)