Picture key base under discussion (7)
I believe the answer is:
tableau
'picture' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tableau that I've seen before include "Scene represented by motionless figures" , "Staged scene" , "pictorial presentation" , "A scene with silent motionless group of people" , "Group of people arranged as for a painting say" .)