Gold backing used by firms for decoration (6)
I believe the answer is:
rococo
'for decoration' is the definition.
(I have seen 'Fanciful decoration' mean 'rococo' so perhaps 'decoration' could also mean 'rococo')
'gold backing used by firms' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'backing' says the letters should be written in reverse.
'used by' says to put letters next to each other.
'firms' becomes 'coco' (I am not sure about this - if you are sure you should believe this answer much more).
'or' written backwards gives 'ro'.
'ro'+'coco'='ROCOCO'
Can you help me to learn more?
(Other definitions for rococo that I've seen before include "Late baroque style (of furniture or architecture)" , "Ornate late baroque style" , "With an ornately decorated style" , "Fanciful decoration" , "Highly ornamental style" .)