Rider volunteers to leave military administrator outside entrance to courtyard (7)

I believe the answer is:

adjunct

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'rider' is the definition.
(I've seen this before)

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

(Other definitions for adjunct that I've seen before include "Person subordinate to another" , "Thing added" , "Incidental extra" , "Something joined or added" , "'Assistant, professor say (7)'" .)

I've seen this clue in The Hindu.

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