Celebrations not beginning, sadly (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'celebrations not beginning' is the wordplay.
'celebrations' becomes 'galas' (gala is a kind of celebration).
'not beginning' means to remove the first letter.
'galas' with its first letter taken off is 'ALAS'.
(Other definitions for alas that I've seen before include "Regrettably, unfortunately" , "woebegone expression" , "Interjection of sorrow" , "Too bad, so sad" , "Lackaday" .)