The most common and most likely answer to this crossword clue is the 10 letter word QUALIFYING
I've seen this crossword clue in The Guardian
Q
U
A
L
I
F
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Buddy explains!
The straight part
We know that "Toning down/F1 practice session" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case toning down/f1 practice session = qualifying.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 QUALIFYING mean?
QUALIFYING verb- To describe or characterize something by listing its qualities.
- To make someone, or to become competent or eligible for some position or task.
- To certify or license someone for something.
- To modify, limit, restrict or moderate something; especially to add conditions or requirements for an assertion to be true.
- To mitigate, alleviate (something); to make less disagreeable.
- To compete successfully in some stage of a competition and become eligible for the next stage.
- To give individual quality to; to modulate; to vary; to regulate.
- To throw and catch each object at least twice.
- A qualification or added condition.
- An examination that must be taken in order to qualify.