Ruler's first-rate son leaves suitably dressed and accompanied (6,5)
I believe the answer is:
caesar salad
'leaves suitably dressed and accompanied' is the definition.
(salad made up of leaves)
'ruler's first-rate son' is the wordplay.
'ruler's' becomes 'caesars' (Julius Caesar).
'first-rate' becomes 'a'.
'son' becomes 'lad' (I've seen this before).
'caesars'+'a'+'lad'='CAESAR SALAD'
(Other definitions for caesar salad that I've seen before include "food" , "Dish containing lettuce" , "Lettuce-based dish" , "Course supplement" , "Seasoned mixture of cos lettuce and croutons" .)