Proposition toff with mournful tone (8)
I believe the answer is:
plangent
'with mournful tone' is the definition.
(plangent can mean sounding mournful)
'proposition toff' is the wordplay.
'proposition' becomes 'plan' (a plan is a proposition or idea).
'toff' becomes 'gent' (I've seen this in another clue).
'plan'+'gent'='PLANGENT'
(Other definitions for plangent that I've seen before include "Resonant, reverberating with mournful sound" , "Loud and unhappy" , "booming" , "Loud and sad" , "Of sounds, resonant and reverberating" .)