The most common and most likely answer to this crossword clue is the 7 letter word WALMART
I've seen this crossword clue in The Guardian
W
A
L
M
A
R
T
More clues leading to the result WALMART
Buddy explains!

The straight part
We know that "US chain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case us chain = walmart.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.