The most common and most likely answer to this crossword clue is the 8 letter word CODEWORD
I've seen this crossword clue in The Guardian
C
O
D
E
W
O
R
D
More clues leading to the result CODEWORD
Buddy explains!

The straight part
We know that "puzzle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case puzzle = codeword.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 CODEWORD mean?
CODEWORD noun- A string representing an encoded piece of text.
- A word or phrase (or by extension a concept) used to make a (concealed) reference to another word or concept.
- A type of crossword puzzle where individual letters are represented by numbers and the solver must identify them by their position and frequency.