Tirade by star, one officiating at ceremony (9)
I believe the answer is:
celebrant
'one officiating at ceremony' is the definition.
'tirade by star' is the wordplay.
'tirade' becomes 'rant' (I've seen this before).
'by' means one lot of letters go next to another.
'star' becomes 'celeb' (I've seen this before).
'rant' after 'celeb' is 'CELEBRANT'.
(Other definitions for celebrant that I've seen before include "One performing a rite" , "person performing rite" , "An officiating priest, especially at Eucharist" .)