Make merry, taking in a show (6)
I believe the answer is:
reveal
'show' is the definition.
(I know that reveal is a more specific form of the action show)
'make merry taking in a' is the wordplay.
'make merry' becomes 'revel' (synonyms).
'taking in' means one lot of letters goes inside another.
'revel' placed around 'a' is 'REVEAL'.
(Other definitions for reveal that I've seen before include "Make public" , "Make disclosure" , "Disclose, make apparent" , "Make known, uncover and allow to be seen" , "Make known to others" .)