The most common and most likely answer to this crossword clue is the 9 letter word DESCALING
I've seen this crossword clue in The Daily Telegraph
D
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More clues leading to the result DESCALING
Buddy explains!

The straight part
We know that "Preparing fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preparing fish = descaling.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 DESCALING mean?
DESCALING verb- To remove scale from the surface of a metal, especially from the inner surfaces of boilers etc.
- To remove the scales from a fish etc.
- The removal of scale from the surface of a metal, especially from the inner surfaces of boilers etc.