Frivolous conduct entertains everyone (7)
I believe the answer is:
shallow
'frivolous' is the definition.
(I've seen this before)
'conduct entertains everyone' is the wordplay.
'conduct' becomes 'show' (showing is a kind of conducting).
'entertains' is an insertion indicator.
'everyone' becomes 'all' (I've seen this before).
'show' enclosing 'all' is 'SHALLOW'.
(Other definitions for shallow that I've seen before include "Frivolous" , "Of water, not deep" , "Lacking depth" , "Of no great depth" , "Not profound" .)