The most common and most likely answer to this crossword clue is the 9 letter word FOOTBRAKE
I've seen this crossword clue in The Daily Telegraph
F
O
O
T
B
R
A
K
E
Buddy explains!
The straight part
We know that "If you need to step on it, don’t step on it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case if you need to step on it, don’t step on it = footbrake.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 FOOTBRAKE mean?
FOOTBRAKE noun- A brake controlled by foot, as opposed to a hand brake.