NLU entity extraction when user gives single word as response

I can do that but how at the backend ill come to know which slot to fill. I may have 5 more slots. Is there any way I can send slot key to fill.

eg.

action->utter_ask_city user-> Delhi

using custom action I can pick Delhi to be slot value, but at backend I dont have that info which slot to fill. in tracker i get only user text and action text , not the slot key making custom actions for all kind of slots doesnt seem to be a feasible solution. I want more generic solution