The most common and most likely answer to this crossword clue is the 7 letter word GANACHE
I've seen this crossword clue in The Guardian
G
A
N
A
C
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Buddy explains!

The straight part
We know that "chocolate filling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chocolate filling = ganache.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 GANACHE mean?
GANACHE noun- A rich sauce, made of chocolate and cream, used also as the filling of truffles, and as a glaze.