Short special event? Certain to be something much appreciated (8)
I believe the answer is:
treasure
'be something much appreciated' is the definition.
(something treasured)
'short special event? certain' is the wordplay.
'short' means to remove the last letter.
'special event?' becomes 'treat' (eg treat could mean a day out).
'certain' becomes 'sure' (synonyms).
'treat' with its final letter removed is 'trea'.
'trea'+'sure'='TREASURE'
'to' acts as a link.
(Other definitions for treasure that I've seen before include "Jewel hoard" , "quantity of wealth" , "Quantity of precious objects" , "Classic adventure story" , "Hoard of jewels and money" .)