Crossword Genius

Enter office uninvited? (5)

Ross

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 can't understand how they can define each 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 "Depose illegally" , "Take power unlawfully" , "Take (power) by force or illegally" , "Expropriate" , "Appropriate without authority" .)

I've seen this clue in The Sun.
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