Festival involving a steer (6)
I believe the answer is:
easter
'festival' is the definition.
(Christian festival)
'involving a steer' is the wordplay.
'involving' is an anagram indicator.
'a'+'steer'='asteer'
'asteer' anagrammed gives 'EASTER'.
(Other definitions for easter that I've seen before include "Card, egg or Island" , "Parade; egg; lily" , "A terse sort of Spring festival?" , "Teaser (anag.)" , "Christian feast of the Resurrection" .)