Suppress encouragement to speed -- it isn't necessary

I believe the answer is:

step on

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'suppress' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.

I don't understand the remainder of the clue.

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(Other definitions for step on that I've seen before include "Behave harshly towards" , "crush" .)

I've seen this clue in The Telegraph.

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