Reveal everyone inside to be superficial (7)
I believe the answer is:
shallow
'be superficial' is the definition.
(similar in meaning)
'reveal everyone inside' is the wordplay.
'reveal' becomes 'show' (synonyms).
'everyone' becomes 'all' (all people).
'inside' is an insertion indicator.
'show' enclosing 'all' is 'SHALLOW'.
'to' is the link.
(Other definitions for shallow that I've seen before include "flimsy" , "justice" , "Not profound" , "incapable of serious thought" , "Having little depth" .)