Celebrate penning a show (6)
I believe the answer is:
reveal
'show' is the definition.
(synonyms)
'celebrate penning a' is the wordplay.
'celebrate' becomes 'revel' (reveling is a kind of celebrating).
'penning' means one lot of letters goes inside another.
'revel' going around 'a' is 'REVEAL'.
(Other definitions for reveal that I've seen before include "Make public" , "'Disclose, divulge (6)'" , "Make known, expose" , "Remove cover, unveil" , "window, in part" .)