Hello! That means winter's here! (4)
I believe the answer is:
hail
'hello' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hail that I've seen before include "Old Roman salute, or frozen pellets" , "Shower of ice pellets" , "Falling ice pellets" , "Say hello to the ice pellets" , "Greet - precipitation" .)