Holiday involving a steer (6)
I believe the answer is:
easter
'holiday' is the definition.
(Easter is a Christian holiday)
'involving a steer' is the wordplay.
'involving' indicates anagramming the letters.
'a'+'steer'='asteer'
'asteer' with letters rearranged gives 'EASTER'.
(Other definitions for easter that I've seen before include "Egg-rolling time" , "A terse sort of Spring festival?" , "Christian celebration of the Resurrection" , "Time to parade" , "Sort of tide" .)