Everything in gallery (3)
I believe the answer is:
'everything' is the definition.
'in gallery' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALL' is hidden within '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" .)