Everything in gallery (3)
I believe the answer is:
all
'everything' is the definition.
(all things)
'in gallery' is the wordplay.
'in' says the answer is hidden in the clue.
'ALL' can be found hidden inside 'gallery'.
(Other definitions for all that I've seen before include "Entire" , "Garden passage between trees or bushes" , "that's as many as there are" , "None excluded" , "Straight garden path" .)