Stupid aspersion in speech, gradually losing force (10)
I believe the answer is:
diminuendo
'gradually losing force' is the definition.
(musical term)
'stupid aspersion in speech' is the wordplay.
I cannot quite see how this works, but
'stupid' could be 'dim' (similar in meaning) and 'dim' is found within the answer.
'in' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for diminuendo that I've seen before include "becoming quieter" , "losing force" , "Decrease in loudness" , "Getting quieter" , "Letting sound fade away" .)