Celebration involving a steer (6)
I believe the answer is:
easter
'celebration' is the definition.
(I've seen this before)
'involving a steer' is the wordplay.
'involving' indicates an anagram.
'a'+'steer'='asteer'
'asteer' anagrammed gives 'EASTER'.
(Other definitions for easter that I've seen before include "Christian festival in Spring" , "Time to celebrate" , "holiday time" , "Important Christian festival" , "A terse sort of Spring festival?" .)