The most common and most likely answer to this crossword clue is the 5 plus 8 letter word NORTH AMERICAN
I've seen this crossword clue in The Guardian
N
O
R
T
H
A
M
E
R
I
C
A
N
More clues leading to the result NORTH AMERICAN
Buddy explains!
The straight part
We know that "Could be a Mexican" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case could be a mexican = north american.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.