Part of pageant I'm at — terrific ephemeral stuff (10)
I believe the answer is:
antimatter
'ephemeral stuff' is the definition.
(matter and antimatter can only co-exist briefly)
'part of pageant i'm at terrific' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ANTIMATTER' can be found hidden inside 'pageant im at terrific'.
(Other definitions for antimatter that I've seen before include "stuff with positrons" , "counterpart to our universe" , "it's potentially explosive" , "Counterpart to everyday substance" .)