Everyone in gallery (3)
I believe the answer is:
all
'everyone' is the definition.
(all people)
'in gallery' is the wordplay.
'in' says the answer is hidden in the clue.
'ALL' is hidden in the letters of 'gallery'.
(Other definitions for all that I've seen before include "and 16 Across and 23 Across: Even so" , "in toto" , "Entire" , "The lot, everyone" , "Garden path (Fr.)" .)