Don leaves San Diego for protection (5)

I believe the answer is:

aegis

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'protection' is the definition.
('aegis' can be a synonym of 'protection')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for aegis that I've seen before include "Protection (originally of Zeus)" , "shelter" , "Kindly endorsement and guidance" , "Protection, backing" , "Backing or support" .)

I've seen this clue in the Irish Times.

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