Celebrate screening a show (6)
I believe the answer is:
reveal
'show' is the definition.
('reveal' can be a synonym of 'show')
'celebrate screening a' is the wordplay.
'celebrate' becomes 'revel' (reveling is a kind of celebrating).
'screening' means one lot of letters goes inside another (I've seen this in other clues).
'revel' enclosing 'a' is 'REVEAL'.
(Other definitions for reveal that I've seen before include "Expose or disclose" , "Expose, divulge" , "Make known to others" , "Show, uncover" , "Make disclosure" .)