The most common and most likely answer to this crossword clue is the 7 letter word NAHUATL
N
A
H
U
A
T
L
Buddy explains!
The straight part
We know that "This originally, native Aztecs happily used as their language" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this originally, native aztecs happily used as their language = nahuatl.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.