Enter the office uninvited? (5)

I believe the answer is:

usurp

I'm a little stuck... Click here to teach me more about this clue!

'enter' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for usurp that I've seen before include "Supplant (someone in power)" , "Commandeer" , "Arrogate" , "Dethrone" , "Assume by force" .)

I've seen this clue in The Guardian, The Telegraph and The Globe and Mail.

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