Put more plainly, tenor and soprano's parting was overdue (9)
I believe the answer is:
translate
'put more plainly tenor' is the definition.
I can't tell whether this definition defines the answer.
'and soprano's parting was overdue' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'soprano' becomes 's' (musical abbreviation).
'parting' indicates putting letters inside.
'was' becomes 'tra' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'overdue' becomes 'late' (similar in meaning).
'n'+'s'='ns'
'tra'+'late'='tralate'
'ns' placed inside 'tralate' is 'TRANSLATE'.
Can you help me to learn more?
(Other definitions for translate that I've seen before include "Change into another language" , "shift" , "make comprehensible" , "Restate in another language" , "Switch language" .)