Where to send a letter to Uncle Sam (3)
I believe the answer is:
usa
This clue might be a double definition.
'where to send a letter' is the first definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'uncle sam' is the second definition.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for usa that I've seen before include "Initially, a great federation" , "11 Nation" , "Trumpland?" , "A great national federation" , "United States" .)