The most common and most likely answer to this crossword clue is the 6 plus 8 letter word SPEEDY GONZALES
I've seen this crossword clue in The Guardian
S
P
E
E
D
Y
G
O
N
Z
A
L
E
S
Buddy explains!
The straight part
We know that "Little Mexican" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case little mexican = speedy gonzales.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.