Enters the office uninvited? (6)
I believe the answer is:
'enters' 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.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for usurps that I've seen before include "Seizes wrongfully or by force" , "Takes a position of power illegally" , "Takes position wrongfully or by force" , "Encroaches upon" , "takes place" .)