First-class setter's rich! (6)
I believe the answer is:
plummy
'rich' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'first-class setter's' is the wordplay.
'first class' becomes 'plum' (I can't justify this - if you can you should believe this answer much more).
'setter's' becomes 'my' (belonging to the crossword setter).
'plum'+'my'='PLUMMY'
Can you help me to learn more?
(Other definitions for plummy that I've seen before include "Of voice, sounding affectedly full - with stonefruit?" , "Choice" , "(Of a voice) typically upper class" , "Rich and resonant" , "Refined" .)