The most common and most likely answer to this crossword clue is the 6 letter word RETARD
R
E
T
A
R
D
More clues leading to the result RETARD
- Check reversing speed in road
- Delay on time machine is ignored
- Embarrassed about sailor’s delay
- Delay caused by extrememly efficient soldiers returning to block road
- Delay 25, firework softly dropped
- Slow blast with Queen in comeback
- Slow down Liverpool fan crossing pitch
- Sailor getting stuck into wine produces delay
- Slow pitch left covered
- Slow time in last day
Buddy explains!

The straight part
We know that "Slow down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slow down = retard.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 RETARD mean?
RETARD noun- Retardation; delay.
- A slowing down of the tempo; a ritardando.
- A person with mental retardation.
- A person or being who is extremely stupid or slow to learn.
- To keep delaying; to continue to hinder; to prevent from progress
- To put off; to postpone.
- To be slow or dilatory to perform (something).
- To decelerate; to slow down.
- To stay back.