Everyone in performance is superficial (7)
I believe the answer is:
shallow
'is superficial' is the definition.
(similar in meaning)
'everyone in performance' is the wordplay.
'everyone' becomes 'all' (all people).
'in' indicates putting letters inside.
'performance' becomes 'show' (I've seen this in other clues).
'all' inserted inside 'show' is 'SHALLOW'.
(Other definitions for shallow that I've seen before include "flimsy" , "Of water, not deep" , "incapable of serious thought" , "Frivolous" , "Not profound" .)