Start of a seasonal greeting (5)
I believe the answer is:
happy
'start of a seasonal greeting' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for happy that I've seen before include "Fortunate - joyful" , "Joyous" , "Contented" , "Fortunate, convenient" , "Pleased, content" .)