Artistic style (6)
(Other definitions for rococo that I've seen before include "Ornate style of decoration." , "Grotesque" , "Fanciful decoration" , "Old style of ornamentation" , "With an ornately decorated style" .)
'artistic style' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for idiom that I've seen before include "Distinctive expression" , "Characteristic mode of expression" , "Group of words in which the meaning may not be obvious" , "Expression that does not make literal sense" , "Peculiarity of phrase in a language" .)