Everyone in gallery (3)
I believe the answer is:
all
'everyone' is the definition.
(all people)
'in gallery' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALL' can be found hidden inside '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.)" .)