Everything in performance is superficial (7)
I believe the answer is:
shallow
'superficial' is the definition.
('shallow' can be similar in meaning to 'superficial')
'everything in performance' is the wordplay.
'everything' becomes 'all' (I've seen this before).
'in' means one lot of letters goes inside another.
'performance' becomes 'show' (I've seen this before).
'all' going into 'show' is 'SHALLOW'.
'is' is the link.
(Other definitions for shallow that I've seen before include "Frivolous" , "incapable of serious thought" , "justice" , "12 [TWO DIMENSIONAL]" , "Insincere" .)