A tree's transformed for the holiday (6)
I believe the answer is:
easter
'the holiday' is the definition.
(Easter is a Christian holiday)
'a tree's transformed' is the wordplay.
'transformed' indicates an anagram.
'a'+'trees'='atrees'
'atrees' is an anagram of 'EASTER'.
'for' acts as a link.
(Other definitions for easter that I've seen before include "Christian celebration of the Resurrection" , "Christian celebration for Teresa" , "Important Christian festival" , "Church festival" , "festival time" .)