The most common and most likely answer to this crossword clue is the 9 plus 2 plus 3 letter word STRUCTURE OF DNA
I've seen this crossword clue in The Guardian
S
T
R
U
C
T
U
R
E
O
F
D
N
A
Buddy explains!
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.