The most common and most likely answer to this crossword clue is the 4 letter word PUTZ
P
U
T
Z
Buddy explains!

The straight part
We know that "US duffer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case us duffer = putz.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PUTZ mean?
PUTZ noun- Fool, idiot.
- Jerk.
- Penis.
- Waste time.
- A decoration or ornament in the Nativity tradition, usually placed under a Christmas tree.
- (Pennsylvania Dutch) To go around viewing the putzes in the neighborhood.