Drop class by the sound of it (6)
I believe the answer is:
lessen
'drop' is the definition.
(thesaurus)
'class by the sound of it' is the wordplay.
'class' becomes 'lesson' (I've seen this before).
'by the sound of it' shows a homophone (sound like).
'lesson' is a homophone of 'LESSEN'.
(Other definitions for lessen that I've seen before include "Cut back or diminish" , "Abate" , "Make smaller or fewer" , "Make smaller for lenses" , "Reduce in number" .)