Firm follows unconventional trade style (3,4)
I believe the answer is:
art deco
'style' is the definition.
(decorative style)
'firm follows unconventional trade' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'follows' is a charade indicator (letters next to each other).
'unconventional' indicates an anagram.
'trade' anagrammed gives 'artde'.
'co' after 'artde' is 'ART DECO'.
(Other definitions for art deco that I've seen before include "1920s/1930s style" , "1920-30s design style" , "1920s and 30s style of art and architecture" , "Early C 20 style" , "Aesthetic style" .)