City on the Ganges
I believe the answer is:
'city on the ganges' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for patna that I've seen before include "Variety of long-grain rice" , "Long-grain rice type" , "City of savoury rice" , "Long-grained rice type" , "Source of long-grain rice" .)