The most common and most likely answer to this crossword clue is the 9 letter word HANDBRAKE
H
A
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Buddy explains!

The straight part
We know that "slower" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slower = handbrake.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 HANDBRAKE mean?
HANDBRAKE noun- A brake in a vehicle, set by hand, and usually locking on until released by hand, enabling its use when parked.
- The mechanism, particularly the locking hand-operated lever in a motor car, which is used to set and release such a brake.
- To apply the handbrake in a vehicle.