The most common and most likely answer to this crossword clue is the 13 letter word EX-SERVICEMAN
I've seen this crossword clue in The Guardian
E
X
-
S
E
R
V
I
C
E
M
A
N
Buddy explains!
The straight part
We know that "veteran in US" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case veteran in us = ex-serviceman.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 EX-SERVICEMAN mean?
EX-SERVICEMAN noun- A man who used to be in the armed forces