Bringing forward time of festival causes problem (6)
I believe the answer is:
teaser
'problem' is the definition.
(I know that teaser is a type of problem)
I cannot understand how the remainder of the clue works.
(Other definitions for teaser that I've seen before include "Difficult problem concerning Easter" , "one using a backcomb" , "One who pulls someone's leg" , "Hard question" , "Easter (anag) - puzzle" .)