Greet with shock, finally changing sides (4)

I believe the answer is:

hail

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'greet' is the definition.
(hailing is a kind of greeting)

I don't understand the remainder of the clue.

(Other definitions for hail that I've seen before include "Greet, call or salute" , "An old Roman salute as to Caesar" , "Frozen rain or Roman greeting" , "Greet - bad weather" , "Old Roman salute, or frozen pellets" .)

I've seen this clue in The Telegraph.

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