Church is smart shifting this festival (9)
I believe the answer is:
christmas
'this festival' is the definition.
(I've seen this before)
'church is smart shifting' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'shifting' is an anagram indicator.
'is'+'smart'='issmart'
'issmart' with letters rearranged gives 'ristmas'.
'ch'+'ristmas'='CHRISTMAS'
(Other definitions for christmas that I've seen before include "Charms sit at this time of year" , "Christian festival" , "Holy festival" , "Yule, Noel" , "&14 Former name of Kiritimati Island" .)