Dad's brother Sam is in the US (5)
I believe the answer is:
uncle
'dad's brother' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for uncle that I've seen before include "Family relation" , "He's related" , "Pawnbroker (sl.)" , ". . . . . Sam, the US government" , "Male relative - pawnbroker" .)