Everything in the performance is superficial (7)
I believe the answer is:
shallow
'superficial' is the definition.
('shallow' can be similar in meaning to 'superficial')
'everything in the performance' is the wordplay.
'everything' becomes 'all' (I've seen this before).
'in the' is an insertion indicator.
'performance' becomes 'show' (I've seen this before).
'all' placed into 'show' is 'SHALLOW'.
'is' acts as a link.
(Other definitions for shallow that I've seen before include "Superficial" , "justice" , "flimsy" , "Of water, not deep" , "Lacking depth" .)