The ultimate opening episode (5)
I believe the answer is:
event
'episode' is the definition.
(thesaurus)
'the ultimate opening' is the wordplay.
'ultimate' indicates one should take the final letters.
'opening' becomes 'vent' (vent is a kind of opening).
The final letter of 'the' is 'e'.
'e'+'vent'='EVENT'
(Other definitions for event that I've seen before include "important" , "special occasion" , "'Occurrence, happening (5)'" , "tournament" , "long jump, say" .)