Everything in gallery (3)
I believe the answer is:
all
'everything' is the definition.
(all things)
'in gallery' is the wordplay.
'in' indicates a hidden word.
'ALL' is hidden in the letters of '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" .)