The highest level of compensation rejected by crown (3,6)
I believe the answer is:
top drawer
'the highest level' is the definition.
(I've seen this before)
'of compensation rejected by crown' is the wordplay.
'of compensation' becomes 'reward' (I can't justify this - if you can you should believe this answer much more).
'rejected' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'crown' becomes 'top' (synonyms).
'reward' in reverse letter order is 'drawer'.
'drawer' after 'top' is 'TOP-DRAWER'.
Can you help me to learn more?
(Other definitions for top drawer that I've seen before include "Highest social position - or sliding furniture box" , "Well-bred" , "high-class" , "Those in high society" , "upper class" .)