One dropped into Manhattan on way to diner, perhaps (3,4)
I believe the answer is:
ice cube
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
I don't understand the rest of the clue.
(Other definitions for ice cube that I've seen before include "Cooling addition to drink" , "Six-faced frozen cooler" , "Something frozen" , "Frozen lump" , "Drink coolant" .)