The most common and most likely answer to this crossword clue is the 5 plus 8 letter word HEAVY HYDROGEN
H
E
A
V
Y
H
Y
D
R
O
G
E
N
Buddy explains!
The straight part
We know that "D" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case d = heavy hydrogen.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.