Make merry, take in a show (6)
I believe the answer is:
reveal
'show' is the definition.
('reveal' can be a synonym of 'show')
'make merry take in a' is the wordplay.
'make merry' becomes 'revel' ('revel' can be a synonym of 'make merry').
'take in' is an insertion indicator.
'revel' going around 'a' is 'REVEAL'.
(Other definitions for reveal that I've seen before include "Expose or disclose" , "Leaver (anag.)" , "'Make known, disclose (6)'" , "Disclose or uncover" , "Disclose, make apparent" .)