Assume you'll lose your footing because of (4,2)

I believe the answer is:

slip on

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'assume' is the definition.
(I know that slip on is a more specific form of the action assume)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for slip on that I've seen before include "Type of garment" , "shoe, perhaps" , "easy to cope with when dressing" .)

I've seen this clue in the Evening Standard.

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