Rail ticket still on the shelf (6)
I believe the answer is:
single
This clue is a double definition.
'rail ticket' is the first definition.
'still on the shelf' is the second definition.
(without a partner)
(Other definitions for single that I've seen before include "Individual - unmarried" , "Concentrating on only one aim" , "Unique" , "Unattached" , "Not one of several" .)