Proceed on a quiet note, that's plain (6)
I believe the answer is:
steppe
'that's plain' is the definition.
(steppe is a kind of plain)
'proceed on a quiet note' is the wordplay.
'proceed on' becomes 'step' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'note' becomes 'e' (musical note).
'step'+'p'+'e'='STEPPE'
Can you help me to learn more?
(Other definitions for steppe that I've seen before include "Large area of unforested grassland" , "Dry grassy plain of eastern Europe and Asia" , "Dry plain of Eurasia" , "Large area of flat grassland in Europe or Siberia" , "Edxtensive plain" .)