Frustrated; that's what a mule is (7)

I believe the answer is:

crossed

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'frustrated' is the definition.
('cross' can be a synonym of 'frustrate')

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

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(Other definitions for crossed that I've seen before include "Went over to the other side" , "Traversed" , "Intersected; opposed" , "Passed" , "Came over and < annoyed" .)

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

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