1920s design (3,4)
I believe the answer is:
'1920s design' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for art deco that I've seen before include "Style of design popular in the 1920s and 1930s" , "Old style" , "Style of 1920s/30s" , "pre-war style" , "Highly geometric style" .)