Mean onion tart cooked up for garnish? (13)
I believe the answer is:
ornamentation
'garnish?' is the definition.
(thesaurus)
'mean onion tart cooked up' is the wordplay.
'cooked up' is an anagram indicator.
'mean'+'onion'+'tart'='meanoniontart'
'meanoniontart' with letters rearranged gives 'ORNAMENTATION'.
'for' acts as a link.
(Other definitions for ornamentation that I've seen before include "Adornment" , "Decorative work" , "Decoration, embellishment" , "Detailed artistic embellishment" .)