Everything in the exhibition is superficial (7)
I believe the answer is:
shallow
'is superficial' is the definition.
(similar in meaning)
'everything in the exhibition' is the wordplay.
'everything' becomes 'all' (all things).
'in the' indicates putting letters inside.
'exhibition' becomes 'show' (I've seen this before).
'all' put within 'show' is 'SHALLOW'.
(Other definitions for shallow that I've seen before include "Of water, not deep" , "Frivolous" , "Of no great depth" , "Insincere" , "Not profound" .)