Everything in the exhibition is trivial (7)
I believe the answer is:
shallow
'trivial' is the definition.
(thesaurus)
'everything in the exhibition' is the wordplay.
'everything' becomes 'all' (I've seen this before).
'in the' indicates putting letters inside.
'exhibition' becomes 'show' (I've seen this before).
'all' put within 'show' is 'SHALLOW'.
'is' is the link.
(Other definitions for shallow that I've seen before include "Of waters, not deep" , "Trivial" , "12 [TWO DIMENSIONAL]" , "requiring no serious thought" , "justice" .)