Frustrated; that's what a mule is (7)
I believe the answer is:
'frustrated' is the definition.
('cross' can be a synonym of 'frustrate')
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for crossed that I've seen before include "Went over to the other side" , "Traversed" , "Intersected; opposed" , "Passed" , "Came over and < annoyed" .)