Make merry outside 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 outside a' is the wordplay.
'make merry' becomes 'revel' (synonyms).
'outside' is an insertion indicator.
'revel' enclosing 'a' is 'REVEAL'.
(Other definitions for reveal that I've seen before include "Remove cover to display" , "Make public" , "Make known, uncover and allow to be seen" , "Tell the truth" , "'Disclose, divulge (6)'" .)