Old style trade ruined company (3,4)
I believe the answer is:
art deco
'old style' is the definition.
(I've seen this before)
'trade ruined company' is the wordplay.
'ruined' indicates an anagram.
'company' becomes 'co' (abbreviation).
'trade' with letters rearranged gives 'artde'.
'artde'+'co'='ART DECO'
(Other definitions for art deco that I've seen before include "Redcoat (anag) -- inter-war design style" , "1920s/1930s style" , "Geometric style of ornamentation" , "Architecture and design movement of the 1930s" , "Architecture and design movement of the 1920s and 30s" .)