Assumes, when one is brought down by (5,2)

I believe the answer is:

slips on

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'assumes' is the definition.
Although both the answer and definition are verbs in their -s form, I can't see how they can define each other.

'when one is brought down by' is the wordplay.
'when' becomes 'on' (I've seen this in other clues).
'one is brought down' becomes 'slips' (I can't justify this - if you can you should believe this answer much more).
'by' says to put letters next to each other.
'on' after 'slips' is 'SLIPS ON'.

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I've seen this clue in the Evening Standard.

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